Add TetGen-based tetrahedral mesh volumes and MRCP input support

[ynskHuh]

Proposed pull request

Title

Add TetGen-based tetrahedral mesh volumes and MRCP input support

GitHub PR body

Purpose

This PR adds support for constructing parameterised Geant4 tetrahedral meshes
from TetGen files, with MRCP-style material and color mappings. It allows a
tetrahedral computational phantom to be used directly without converting it
to an image-volume representation.

Main changes

• Add TetrahedralMeshVolume and its bounding-box solid.
• Add a C++ parameterised mesh builder that creates one G4Tet per TetGen
  element and assigns materials, colors, visibility, and copy numbers by
  region.
• Add strict TetGen node and element validation, including support for complete
  zero-based and one-based node indices.
• Interpret MRCP node coordinates as centimetres at the C++ input boundary and
  convert them to Geant4 units with a fixed conversion.
• Parse MRCP material density and elemental mass fractions and construct the
  corresponding Geant4 materials.
• Add optional region selection through keep_regions and fallback handling
  through default_material.
• Add CopyNumberFilter so actor scoring can select parameterised
  tetrahedron copies.
• Add synthetic non-anatomical TetGen datasets and a minimal visualization
  example under opengate/contrib/mrcp.
• Add user-facing documentation in opengate/contrib/mrcp/README.md.

Implementation map

The following map identifies the files changed by this PR and the main
classes or functions reviewers may want to inspect.

C++ tetrahedral mesh implementation

• core/opengate_core/opengate_lib/GateTetrahedralMeshParameterisation.h
  • Declares GateTetrahedralMeshParameterisation and the public TetGen mesh
    builder entry points.
• core/opengate_core/opengate_lib/GateTetrahedralMeshParameterisation.cpp
  • read_node_file() validates the TetGen header, node count, duplicate IDs,
    and complete zero-based or one-based index ranges. The caller supplies the
    coordinate scale used during parsing.
  • read_ele_file() reads tetrahedral connectivity and region IDs.
  • GateTetrahedralMeshParameterisation::ComputeSolid() selects the G4Tet
    solid for each parameterised copy.
  • GateTetrahedralMeshParameterisation::ComputeMaterial() assigns the
    material associated with the tetrahedron's region.
  • GateTetrahedralMeshParameterisation::get_or_create_vis() creates and
    reuses region-specific visualization attributes.
  • build_tetrahedral_mesh_impl() constructs the shared parameterised
    physical volume.
  • build_tetrahedral_mesh_from_tetgen() retains configurable scaling for
    generic TetGen input.
  • build_mrcp_tetrahedral_mesh_from_tetgen() applies the fixed MRCP
    centimetre-to-Geant4 conversion.
  • build_tetrahedral_mesh_from_tetgen_material_names() provides a builder
    variant that resolves existing Geant4 materials by name.

C++ and pybind11 registration

• core/opengate_core/opengate_lib/pyGateTetrahedralMesh.cpp
  • init_GateTetrahedralMesh() exposes the generic, MRCP-specific, and
    material-name mesh builders to Python.
  • rgba_to_colour_map() converts Python region/RGBA mappings to Geant4
    colors.
• core/opengate_core/g4_bindings/pyG4Tet.cpp
  • init_G4Tet() exposes G4Tet construction and selected solid methods.
  • tet_is_degenerate() provides a geometric tetrahedron degeneracy check.
• core/opengate_core/g4_bindings/pyG4Material.cpp
  • Exposes both G4Material::AddElement() overloads so Python can add
    elements by mass fraction or atom count.
• core/opengate_core/opengate_core.cpp
  • Registers the new G4Tet, tetrahedral mesh, and copy-number filter
    bindings in the opengate_core module.

Python geometry interface

• opengate/geometry/solids.py
  • Adds TetrahedralMeshEnvelopeSolid.
  • _read_node_bounds() skips and validates the TetGen header before reading
    coordinates and verifies the declared node count.
  • get_bbox_size_and_center_mm() converts the MRCP node bounds from
    centimetres to millimetres and calculates the enclosing box.
  • build_solid() creates the enclosing G4Box with a configurable margin.
• opengate/geometry/volumes.py
  • Adds TetrahedralMeshVolume and its user properties.
  • _parse_mrcp_material_file() reads region IDs, densities, and elemental
    mass fractions from the MRCP material format.
  • _ensure_custom_material_from_zfrac() creates and caches the resulting
    G4Material objects.
  • _parse_colour_dat() reads region RGBA values and visibility.
  • _build_region_dicts() applies keep_regions and assembles the mappings
    passed to C++.
  • construct() builds the envelope and invokes the MRCP-specific C++ mesh
    builder.
• opengate/managers.py
  • Imports and registers TetrahedralMeshVolume, making
    sim.add_volume("TetrahedralMesh", ...) available.

Copy-number filtering

• core/opengate_core/opengate_lib/filters/GateCopyNumberFilter.h/.cpp
  • Adds GateCopyNumberFilter.
  • InitializeUserInfo() reads and deduplicates copy_numbers.
  • Evaluate() tests the pre-step touchable copy number at history depth
    zero.
• core/opengate_core/opengate_lib/filters/pyGateCopyNumberFilter.cpp
  • init_GateCopyNumberFilter() exposes the C++ filter to Python.
• opengate/actors/filters.py
  • Adds and registers the Python CopyNumberFilter interface.

Contribution and example data

• opengate/contrib/__init__.py
  • Registers the mrcp contribution package.
• opengate/contrib/mrcp/mrcp_simple.py
  • Provides a minimal geometry and Qt visualization example.
• opengate/contrib/mrcp/simple.*
  • Provides the synthetic sphere, cube, and regular-tetrahedron input.
• opengate/contrib/mrcp/letter.*
  • Provides the separate synthetic 3D GATE input.
• opengate/contrib/mrcp/README.md
  • Documents input assumptions, volume configuration, example datasets, and
    copy-number filtering.

Why the unit conversion is fixed

MRCP .node coordinates are stored in centimetres, whereas OpenGATE/Geant4
geometry is represented in Geant4 length units. The conversion is performed
while C++ reads the node file. It is intentionally not exposed as a JSON or
Python scale parameter, preventing accidental modification of an input-format
property and keeping generic TetGen scaling behavior separate.

Example data

No anatomical MRCP phantom data are included. The example uses synthetic
geometry:

• simple.*: separated sphere approximation, cube, and regular tetrahedron;

The example only initializes geometry and visualization, so the expected
warning about a missing particle source is harmless.

[tbaudier]

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