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.