[RM-8369] [api] Handle multiple LSF in AbstractSpectrumReader

Created on 2023-09-27 15:11:29 by Thomas Bedrine. % Done: 100

Handling LSF in AbstractSpectrumReader isn't yet compatible with multiobs cases, and it is problematic with upcoming converted spectra from Euclid format (red and blue observations each have their own LSF for the same spectrum).
lsf_data should be a Container -> store multiple lsf
Remove API exception "Multiple LSF not handled" and choose arbitrarily one lsf to provide to the library
AbstractSpectrumReader.get_lsf should have an obs_id argument