Fragpipe parser produces semantically invalid ProForma sequences

[levitsky]

Proteoform sequences constructed from Fragpipe output currently look like this: LHM[147]TNQNMEK-[17]/3

This is not valid in the sense that these modifications cannot be resolved and their mass information is not available, so calculating the mass of this sequence is not possible. Using a proper "mass modification" (e.g. M[+15.9949]) would be desirable.