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

Current limitations: no explicit handling of N/C terminal modifications