Refactor spectrum

Sorry about it but this is big PR.

Overview

• remove inheritance from Spectrum to larch Group
• rework a bit from_dict / to_dict Spectrum functions
• rework Spectra
  • spectra can now be provided to the constructor as a numpy array of Spectrum
  • simplify ROI application to the spectra
• several minor fix
  • fix issue with plot and bkg
  • replace several Spectrum().load_from_dict by Spectrum.from_dict
  • fix some outdated _NexusDatasetDef
  • remove plots from register_process since data will be provided to daquiri another way