split apart dependencies of vaccine peptide selection

* added unit tests for individual ranking components
* vaccine peptide sorting by four criteria (mutant epitope score, negative wildtype epitope score, number mutated residues, distance of mutation start the edge of vaccine peptide)
* print and write FASTA of vaccine peptides with a little mutation metadata
* print epitopes associated with each mutation
Awesome docstring! Super clear. I think it could be made even a little more clear with the following:

T[Q]ADMAAQTTKHKWEAAHVAEQLRAYLEGTCVEWLRRYLENGKETLQR (bracket around modification)

Two such windows:

TQADMAAQTTKHKWE
QADMAAQTTKHKWEA

Yielding a SourceSequence of length 16: ...