feat: Add gravity solver to pygeoos-tools

[bd713]

This PR introduces a comprehensive suite of Python utilities for running and validating gravity simulations using GEOS (see corresponding GEOS PR). These tools support inversion workflows, ensure correctness of forward and adjoint implementations, and provide reproducible testing.

The PR includes:

• GravitySolver.py
  Extends the base Solver class with gravity-specific functionality, enabling modeling of gravity responses from density models.
• GravityLinearOpSolver.py
  A SciPy-compatible LinearOperator wrapper around the GEOS GravitySolver.
  Supports forward modeling, adjoint modeling, and loss/gradient computation for optimization and inversion workflows.
• InversionUtils.py
  Utility functions for inversion workflows, including:
  • computeResidual, computeL2Loss
  • gradientTest (Taylor expansion validation)
  • adjointTest (inner product symmetry check)