Add control over spherical harmonics expansion degree

[ruboerner]

Purpose

This pull request introduces enhancements to three functions igrf, igrf_gc, and igrf_V within the repository. These changes provide users with greater control over the spherical harmonics expansion by allowing them to specify the minimum and maximum degree of the expansion.

Key Changes

New Arguments:

• Added two new optional arguments (min_degree and max_degree) to the modified functions.
• These arguments enable users to compute:
  • The pure dipole field by setting min_degree=1 and max_degree=1.
  • The multi-pole contribution by adjusting these parameters to desired values.

Improved Flexibility:

Users can now isolate specific contributions to the global magnetic field of the Earth.

Backward Compatibility:

Default values ensure that the original behavior of the functions is preserved when the new arguments are not provided.

Testing Instructions

Pull this branch and install the updated package.

Test the modified functions with various min_degree and max_degree combinations:

• Example 1: min_degree=1, max_degree=1(pure dipole field).
• Example 2: min_degree=2, max_degree=13 (multi-pole contributions).
• Example 3: Default behavior (no arguments).

Verify that results match expectations for the original behavior when no new arguments are provided.

Please provide feedback or suggest further refinements if needed.

[smithara]

Thank you for the contribution! It makes sense. I will check it and merge it in soon