Refactor: Address various items of technical debt

[laleph]

This commit introduces a series of refactorings and improvements to address identified technical debt across the codebase.

Key changes include:

1. Parameter Management (defs.py):

   • Global simulation parameters (strength, nres, offset, nlines) are now encapsulated in a SimParams dataclass for better organization and clarity. Usage in defs.py and test_defs.py has been updated.

2. Type Hinting and Logic (defs.py):

   • The parallel parameter in the integration function and the related pa_val from ring_calculation are now consistently handled as floats (representing a single parallelism value for p=0.1), simplifying logic and removing ambiguity. Type hints and tests were updated accordingly.

3. Test Suite Health (test_defs.py):

   • Previously commented-out tests have been uncommented, reviewed, and fixed. This includes tests for Ring class instantiation with plotting, parallelism edge cases, and active_volume calculations (with corrected expected values). The test suite is now more comprehensive.

4. Code Clarity (defs.py):

   • The ring_calculation function now returns a RingCalculationOutput dataclass instance instead of a large, unnamed tuple, significantly improving readability and maintainability. Consumers of this function (Ring class, tests, notebooks) have been updated.
   • Obsolete commented-out code blocks have been removed.

5. Documentation (defs.py):

   • Comprehensive docstrings have been added or improved for all major functions and classes (SimParams, RingCalculationOutput, Ring, find_separatrix, add_lengths_to_df, parallelism, active_volume, field_plot, ring_calculation, integration).
   • The choice of ODE solver (RK45) in integration is now documented, and the related TODO comment about DOP853 has been removed.
   • A minor bug (potential division by zero) was addressed in parallelism for empty iterables, and this behavior documented.

6. Jupyter Notebooks:

   • ring_magnetic_field.ipynb: Hardcoded parameters and filenames have been replaced with variables defined in a setup cell for easier modification. A TODO comment was addressed by using sim_params. Imports were corrected, and complex indexing logic was clarified with comments.
   • taylor_expansion.ipynb: Markdown cells were added to explain the purpose and steps of the symbolic calculations, improving the notebook's utility as a reference.

These changes aim to make the codebase more robust, maintainable, understandable, and easier to extend.