peptidewithsetmods get hashcode bug

[trishorts]

you can make peptidewithsetmods without digestion params. if you do that and then call gethashcode you get a crash becuase digestionparames is null.

public override int GetHashCode() { return FullSequence.GetHashCode() + DigestionParams.Protease.GetHashCode(); }

can we check if null and return fullsequence hashcode as fallback?

[zrolfs]

#428