[FEATURE] Save out spectra from region of interest

[AlanRace]

The ability to save out the intensities (and corresponding spectral channels) along with the pixel coordinates to CSV will enable further processing outside of SpectralAnalysis. This should be implemented for the whole dataset, or one or more regions of interest.

Will need to automatically detect whether the data has a consistent spectral axis, and if not then ask the user whether they require zero filling to be performed, or store each spectrum with its spectral channels (so each spectrum can have a different length).