Feature/aa sequence tpp

- parse modifications in TPP format:

- allow parsing of modified peptides in TPP/SpectraST format with
  integer (PEPT[181]IDE) or accurate (PEPT[181.004]IDE) masses
- allow parsing of modified peptides in TPP/SpectraST format with
  integer (PEPT[+80]IDE) or accurate (PEPT[+79.96632]IDE) mass
  differences

Authors: @hroest and @grosenberger

Current limitations: 
- no explicit handling of N/C terminal modifications
- breaking the old behavior of OpenMS which would read PEPT[181]IDE as a 8 AA peptide with a T of mass 101.04768 _and then_ another unknown amino acid of mass 181 which does not correspond to any use of the bracket notation that I am aware of outside of OpenMS