Allow for color adjustment of the automatically generated target decoy plots

[Matthias313]

Dear developers,

Thank you very much for providing such very nice software for FDR computation and scoring. We noticed that the current color scheme of the automatically generated FDR scoring plots (targets in green and decoys in red) might could be improved, enabling a more color-blind friendly color scheme e.g. by using red and blue and similar distinguishable colors.

Thanks in advance and also for developing and maintaining the pyprophet software!

Best regards Matthias