pyprophet: support for MAYU export

This pull request enables MAYU export functionality for pyprophet.

The flag --export.mayu generates three files: A FASTA based on all extracted peptides, a result table in MAYU format with 1 - m_score as discriminant score, and a cutoff file with the target - decoy ratio as parameter for MAYU.

MAYU can be run afterwards as following:
Mayu.pl -B test_mayu.csv -C test_mayu.fasta -E DECOY -runR -PmFDR -P protFDR=0.05:t -H 200 -G 0.20 -F $(cat test_mayu.cutoff)