[ENH] Add geophysics module with gravity modeling and magnetic implementation plan

[Leguark]

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.

[Leguark]

• [ENH] Add magnetic forward modeling for Total Magnetic Intensity (TMI) calculation #39
• [ENH] Add geophysics module with gravity modeling and magnetic implementation plan #38 👈 (View in Graphite)
• [FIX] Fix tensor concatenation to handle mixed numpy/torch arrays #37
• main

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[Leguark]

Merge activity

• Oct 27, 12:51 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Oct 27, 12:52 PM UTC: Graphite rebased this pull request as part of a merge.
• Oct 27, 12:55 PM UTC: @Leguark merged this pull request with Graphite.