Add Dz0SCF reference implementation

[Betty0621]

Summary

• add the Dz0SCF effective-potential construction
• support both symmetry-adapted and non-symmetry ROKS references
• provide a fixed-orbital high-spin reference-energy evaluator
• preserve compatibility with the existing NTTDA interface
• add numerical regression tests for SVWN and B3LYP

Energy convention

• mf.e_tot is the variational Dz0SCF energy
• mf.high_spin_energy() evaluates the high-spin ROKS energy
  on the optimized Dz0SCF orbitals
• NTTDA state energies are constructed as
  E_high_spin + omega

Tests

• SVWN and B3LYP Dz0SCF energies
• high-spin reference energies
• Dz0SCF-to-NTTDA integration

Current limitation

Analytic nuclear gradients are not implemented for Dz0SCF.