Feature/peptide hit protein locations

Previously, PeptideHits references were able to reference multiple protein accessions but the associated avidence (AABefore, AAAfter) could only be stored partially (once per PeptideHit). The location in the protein has so far been neglected.
To increase compatibility with MzTab/MzIdentML and to enable more structure proteomics / PTM applications (typically require the position in the protein) the PeptideEvidence class is introduced.
Every PeptideHit now stores 0..n PeptideEvidence objects which the protein accession, position in the protein, preceeding/following amino acid.
could you wrap this as 
```
 libcpp_vector[PeptideHit] getReferencingHits(libcpp_vector[PeptideHit], libcpp_set[String] &) nogil except +
```
instead of void, otherwise this function is kind of pointless