Fix: py_spec handling for mvol=1

[missing-user]

This PR handles some edge cases I encountered in the py_spec Utilities/pythontools/py_spec/output module and plotting library, specifically surrounding equilibria with nvol=1. In that case, quantities like pressure return a scalar instead of a 1 element array, which isn't correctly handled in the plotting methods.

• Handle plotting for equilibria with mvol=1
• Consistent handling of lsurf parameter (some methods accepted scalars and numpy arrays, some threw exceptions when out of range, some didn't... ). Now all methods have the same error handling for lsurf.
• Fixed type annotations to methods such as get_surface_current_density, because lsurf expected a numpy array and not an integer!

[smiet]

Thank you very much for your contribution, it is highly appreciated!

The python wrappers could indeed use some work, and I am very happy to help with any future changes you would want or need.

[missing-user]

Thanks a lot! I'm doing some stellerator optimization with SPEC and SIMSOPT, and will be fixing the issues I encounter as I go along.

I do have one potential bug, that I'm pretty confused about at the moment, but I'll open a separate issue for that.