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.
@timosachsenberg: Why is that a static member function and not a common method? I would find it more intuitive to write `peptide_hit.extractProteinAccessions()` to extract the protein accessions from my peptide hit. It's not like you wouldn't need a PeptideHit object anyway to do that.