Adding support for umbilic curves

[singh-jaydeep]

This PR is meant to update the umbilic curve PR and address some of the TODOs listed there. The changes are based on my understanding of the paper/original PR, so if anything isn't quite right I can revert it.

Overview:

• Cosmetic changes (update docstrings, make variables uniform, removing some unused functions)
• Rename base UmbilicCurve class to FluxSurfaceCurve, allowing for future representations of curves which by construction lie in a flux surface. Subclasses include FourierUmbilicCurve, which is the particular parameterization of umbilic curves used in the original paper.
• Add a parameter m_umbilic to FourierUmbilicCurve, so that the class can represent all curves in the original paper.
• Rename NFP_umbilic_factor to n_umbilic when used internally in FourierUmbilicCurve, and to N_scaling when used externally in basis and grid classes. Wanted a name which doesn't require knowing about umbilic curves when just working with bases or grids.
• Separate compute functions for some of the operations happening in the UmbilicHighCurvature objective.
• Add tests for the curve class and objective.

Notes:

• when we update an i/o attribute, is there a way to regenerate the test examples so it doesn't throw warnings when I run the tests? To avoid those warnings I didn't include N_scaling in i/o attributes for grid and basis, and left if for the curve itself to update these in its _set_up().