pitch_angle_analysis module for analysing sweeps

[fongkenrui]

[WIP] Module for analysing sweep data for pitch angle diagnostic design.

• Implements a class SweepDataset that loads and stores Scotty output data in an xarray Dataset class attribute
• DataArrays have common dimensions of frequency, poloidal angle and toroidal angle (plus extended dimensions)
• Analysis sequences to be implemented as separate class methods; saves analysis output in self.dataset once called
• Methods to be memoized where possible
• Plot methods separate from analysis methods

Update [4/7/2023]

• Implements 2 classes, SweepDataset which loads and interpolates sweep data from Scotty outputs, and PitchDiagnostic, which represents a particular toroidally/poloidally aligned array of antennae.
• PitchDiagnostic can be passed a SweepDataset class to simulate iterated backscattered signal measurements. Assumes a gaussian noise profile.
• Both classes can export to and read from a netCDF file (though repeatedly reading and writing to the same file ends up triggering KeyErrors for reasons I haven't figured out)

[fongkenrui]

Looks good to me! I've left a few comments, some of which you might prefer to ignore. You should also consider adding tests where you can

Thanks for the comments! I've implemented most of the suggested changes that are convenient to make, though I think I'll hold off using the DataTree package for now.