[FEATURE] allow proper handling of heavy/light in TraML

- change headers to include PeptideGroupLabel and LabelType (remove
  GroupLabel which was ambigous)
- [NOP] fix style issue
- [FEATURE] finish TraML heavy/light feature
- GroupLabel header will now be ignored
- PeptideSequenceGroups according to MS:1000893 are now checked whether
  they contain the same peptide sequence and if so, can be fixed (with
  an optional override)
[TEST] tests work

- adopt tests to reflect changes made in TransitionTSVReader
[CLEANUP] store peptide group label as string
[FEATURE] store peptide group label also in LightPeptide
[DOC] adopt and extend documentation