Feature/deflections operate jax

[Jammy2211]

This pull request refactors how lensing calculations are performed in the codebase, consolidating operations previously handled by OperateDeflections into a new class, LensCalc. The changes update class inheritance, function signatures, and usages throughout the codebase and tests to use LensCalc for lensing calculations such as magnification and deflections. Additionally, the pull request simplifies and clarifies the handling of multi-plane lensing and updates test fixtures and expectations accordingly.

Refactoring and API changes:

• Replaced OperateDeflections with LensCalc across the codebase, including imports, class inheritance, and function signatures [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12].
• Updated usage of lensing calculations to use LensCalc methods, including in tracer_util.py, point fitting, and solver logic [1] [2] [3] [4].

Multi-plane lensing and magnification:

• Refactored multi-plane magnification calculations to use LensCalc.from_tracer with explicit use_multi_plane and plane_j arguments, removing the confusing deflections_func property and related logic [1] [2] [3] [4].
• Simplified mock and test tracer classes, removing unused or redundant methods and updating logic to match the new calculation flow [1] [2] [3] [4] [5].

Test and fixture updates:

• Updated tests and fixtures to use the new LensCalc-based API, including changes to expected results due to the refactor and improvements in calculation accuracy [1] [2] [3] [4] [5].
• Adjusted test assertions and mock objects to align with the updated calculation logic and expected outputs.

Cleanup and documentation:

• Removed unused imports and properties, and updated documentation references to reflect the new LensCalc class [1] [2] [3].

These changes modernize and clarify the codebase's approach to lensing calculations, making the API more explicit and maintainable.