Single-stage optimization with spec: CoilSet, ReducedCoilSet and CoilNormalField (PR 2 of 2)

[smiet]

This PR adds new features, and enables the optimizations presented at the Simons' retreat and meeting, and available in preprint (https://arxiv.org/abs/2407.02097).

The main additions are:

class CoilSet

A helper class to handle a set of coil objects as a single object:

• convenience class
• provides helper methods for all the coil-related optimization targets
• serves as the base class for ReducedCoilSet
• allows the ReducedCoilSet and CoilNormalField to be implemented with minimal code duplication.

class ReducedCoilSet

Identical in use to CoilSet, but it's degrees-of-freedom are linear combinations of the coil degrees-of-freedom.

• Calculates its degrees-of-freedom by performing a singular-value-decomposition on an arbitrary user-provided mapping.
• Ability to re-calculate the reduced basis when changing the function or other relevant properties of the CoilSet.
• Removes some overdetermination inherent in direct-from-coil optimization.
• Reduces the dimensionality of the optimization problem, speeds up optimization.

class CoilNormalField

Identical in use to a NormalField and interfaces with SPEC, but has degrees-of-freedom specified by a CoilSet or ReducedCoilSet.

• Provides the normal field on a boundary in the SPEC conventions.
• Uses a cache to avoid redundant Fourier transformations.
• Handles both CoilSet and ReducedCoilSet, provides method to reduce the CoilSet using the mapping to SPEC input parameters (normal field Fourier components).

With these changes the functionality I describe in my preprint will be added to the simsopt master branch. I look forward to your suggestions to improve my code.