[ENH] Add geophysics module with gravity modeling and magnetic implementation plan (#38)

# Add Geophysics Module Documentation and Gravity Benchmarks

This PR adds comprehensive documentation for the geophysics module and introduces analytical benchmark tests for gravity calculations.

## Documentation Additions
- Add detailed README for the geophysics module explaining architecture, implementation, and usage
- Document gravity implementation with mathematical basis and workflow diagrams
- Outline planned magnetics implementation with physical principles and roadmap
- Include developer guide for magnetics implementation with phased approach

## Testing Improvements
- Add analytical benchmark test comparing gravity calculation against sphere solution
- Implement line profile symmetry test to verify spatial accuracy and physical consistency
- Validate numerical accuracy against theoretical expectations
- Add expected values to existing gravity test

The benchmark tests ensure the implementation is both mathematically correct and physically plausible, with appropriate tolerances for voxelization effects.

Author: Leguark