Clamped molecular potentials in spherical coordinates

[haakoek]

Summary
This pull request adds support for constructing clamped molecular potentials in spherical coordinates using both an analytic quadrature-based multipole expansion and a Poisson-based radial construction. It also includes the supporting spherical-coordinate utilities introduced yesterday, along with tests and an example comparing the two constructions for a diatomic potential on the z axis.

Implementation

• Add shared spherical-harmonic utilities in grid_lib.spherical_coordinates.utils.
• Add cartesian_to_spherical for converting Cartesian nuclear positions to spherical coordinates.
• Add clamped_molecular_potential_quadrature to compute V_LM(r) from the analytic kernel r_<^L / r_>^(L+1) for general 3D nuclear positions.
• Add clamped_molecular_potential_Poisson to compute V_LM(r) from a precomputed Poisson-based radial Coulomb tensor W.
• In the Poisson-based helper, use the nearest radial grid point when a nuclear radius is off-grid and issue a warning in that case.
• Export the new helpers from grid_lib.spherical_coordinates and top-level grid_lib.
• Remove redundant imports/comments in the spherical angular-momentum module.

Testing

• Add tests for cartesian_to_spherical.
• Add tests for clamped_molecular_potential_quadrature for centered and displaced nuclei.
• Add tests for clamped_molecular_potential_Poisson covering assembly from W and warning/nearest-grid behavior.
• Verify the new example script compiles and runs.

Example

• Add examples/clamped_molecular_potential_quadrature_vs_poisson.py, which compares quadrature and Poisson radial components for a diatomic potential on the z axis using a FEMDVR grid.