Add monopole electrostatic potential and field with unit tests

[chinmaybhardwaj711]

This PR introduces initial support for electrostatic monopole contributions in the electrostatics module.

Added:

• monopole_potential(q,r_vec)
• monopole_field(q, r_vec)

Features:

• Uses atomic units
• Handles r -> 0 safely
• Fully tested

Tests:

• Added test_electrostatics.py
• All existing test passed (9 passed)

[chinmaybhardwaj711]

@FarnazH @ccastilloo
Happy to implement dipole contributions next in a separate PR if this approach aligns with the project.

[harshjha15335]

Thanks for the clean implementation! One thing worth discussing for future compatibility: this function handles a single charge q and single position r_vec, but as we extend toward a full multipole engine, we'll likely need to evaluate potentials from N atoms at M field points simultaneously. PR #5's dipole/quadrupole functions use a vectorized (N, 3) shape for positions. Would it be worth aligning this monopole implementation to the same convention now — e.g., r_vec of shape (M, 3) and q of shape (N,) — so all multipole terms compose consistently?