Near axis refactor

[Agastyarathee]

Summary

This PR extracts the near_axis class (approx. 600 lines) from essos/fields.py into a new, standalone repository: pyQSC_JAX.

Changes Made

• Refactored fields.py: Removed the near_axis class and replaced it with a detailed ImportError that guides users to the new package.
• Updated Imports: Modified tests/ and examples/ files (e.g., test_near_axis.py, stella_stability.py) to import from the external pyqsc_jax package.
• Preserved Core Functions: Ensured BiotSavart and BiotSavart_from_gamma remain safely intact within the native ESSOS environment.

Testing

• Verified standalone installation of pyQSC_JAX via pip.
• Checked local examples to ensure JAX compilation and plot generation still function correctly with the split architecture.

[rogeriojorge]

@Agastyarathee, please make a pull request into the eg/analysis branch (instead of into the main branch) so that we can review your changes only.