Add possibility to load in STIX spectrograms outputted from IDL (for ospex)

[natsat0919]

Describe the feature

Currently Sunxspex can only handle spectrograms directly from the data centre, however one can also get a spectrogram outputted by stx_convert_spectrogram IDL routine. This spectrogram contains some updated corrections and uncertainty analysis that are not yet implemented to Sunxspex. It would be good to have an option for a user to be able to load in these spectrograms as well.

Proposed solution

Simply add another function in addition to _get_spec_file_info inside stix_spec_code.py that will be able to load in these spectrograms. The loading is very similar for both the data centre and IDL spectrograms, just some minor changes to keyword formatting and time and counts loading.