Spec refactoring/improvement (PR 1 of 3 for single-stage)

[smiet]

Updates to Spec and NormalField and a few helper functions .

Spec class has been updated with the following features:

• detection when free-boundary solution is not converged
• re-set to initial state when not converged or not in force balance
• re-written loops pythonically where possible (for vol in range(self.inputlist.mvol+1) -> `for surface in self.initial_guess:')
• implemented array_translator to further reduce lists over index counters.
• activate_profile: make a profile of your spec equilibrium optimizable in simsopt

The NormalField class has been augmented to:

• include the boundary on which it is defined
• store the Fourier components that describe it in array-form
• include getters and setters for the FCs in array form
• set the dofs in their entirety even when setting single (fixes bug where changing a DOF did not trigger recompute bell)

The Surface base class has two methods: fourier_transform_field and inverse_fourier_transform_field to efficiently calculate fourier transforms in the VMEC convention (-ntor to ntor toroidal components, 0:mpol poloidal). Allows for different Fourier normalization conventions through the 'normalization' parameter.

This is PR 1 of 3, the following two:

• CoilSet and ReducedCoilSet: helper classes to enable interchanging of coils, and create new optimization variables through SVD on coil DOFS
• CoilNormalField and misc changes: class inheriting form NormalField that interfaces to the CoilSets, all other misc changes needed to make the single-stage optimizations work.