Add GPU density-fitted CASCI and diagonal-Hessian CASSCF

[wtpeter]

Summary

This PR adds initial GPU support for density-fitted CASCI and CASSCF.

The main additions are:

• An in-core GPU direct_spin1 FCI solver with GPU Hamiltonian contraction, Davidson diagonalization, and RDM construction.
• GPU density-fitting AO-to-MO integral transformation through DF.ao2mo.
• DFCASCI support for restricted HF and KS references.
• A diagonal-Hessian DFCASSCF orbital optimizer.
• Timing information for AO2MO, effective one-electron integrals, FCI, RDM construction, orbital derivatives, and orbital rotations.
• Unit tests and a CPU/GPU DF-CASSCF comparison example.

The CASSCF orbital step uses a positive diagonal-Hessian model with a common level shift. The final orbital rotation parameters are limited by max_stepsize without increasing the level shift solely to enforce the step limit.

Current limitations

• Only density-fitted CASCI/CASSCF is supported.
• Only restricted HF/KS references are supported.
• The GPU FCI solver is in-core only and raises MemoryError when GPU memory is insufficient.
• CASSCF uses an approximate diagonal Hessian rather than the full augmented-Hessian optimizer.
• Canonicalization, natural orbitals, frozen orbitals, symmetry-adapted FCI, and state-averaged CASSCF are not implemented.

This implementation is intended as a lightweight GPU CASSCF fallback rather than a replacement for the more robust second-order CPU optimizer.