The BaseSource method get_combined_spectra is sort of broken

[DavidT3]

It will not work for single skytile combined TMs, because it always assumes that the obsid is 'combined'

This will be relevant when I rejig the get_combined_lightcurves method as well.

I've lost track of whether I already fixed this problem once and then my dodgy git merges/rebases wiped it out, but we need a 'stacked_spectrum' argument in get_combined_spectra or something.

Should also have a convenience function that checks if there is only one observation, and if so then any combined-instrument products are returned in the place of combined ObsID products.