Warnings for selenocystein (U) and pyrrolysin (O)

[Stortebecker]

Hi.
I saw that the mw function features masses for selenocystein (U) and pyrrolysin (O). However, the aaCheck function does not accept those. Therefore, mw ("U") throws a warning Sequence 1 has unrecognized amino acid types. Output value might be wrong calculated. Actually, it throws the warning twice (?).
Maybe we should avoid the warning for U and O or use a specific one, like sequence contains x untypical amino acids (selenocystein/pyrrolysin).

[dosorio]

Thanks for the advice. I will work on that.