Add direct spherical dipole and quadrupole potential evaluation

[harshjha15335]

Summary

This PR adds direct electrostatic potential evaluation for spherical multipole moments up to quadrupoles.

Added

• spherical_dipole_potential()
• spherical_quadrupole_potential()

Validation

Added tests comparing direct spherical evaluation against the existing Cartesian implementations through spherical-to-Cartesian conversion.

Verified that:

• Direct spherical dipole potential matches Cartesian dipole potential.
• Direct spherical quadrupole potential matches Cartesian quadrupole potential.
• Cartesian ↔ spherical conversion routines remain consistent.

Testing

pytest -v

All tests pass successfully.

[harshjha15335]

@ccastilloo

[harshjha15335]

@ccastilloo

[ccastilloo]

@harshjha15335 I would add a total potential and total field in the spherical formulation as same as you did for cartessian