selenocysteine and pyrrolysine not supported in calculateFragments

[guideflandre]

The 21st and 22nd amino acids selenocysteine (U) and pyrrolysine (O) are not supported in PSMatch. NA's are thus introduced in generated theoretical fragments.

This is a small issue that rarely occurs, but it happened to me and I thought it might be interesting to add their masses/characteristics into calculateFragments / getAminoAcids.

[lgatto]

I don't think that adding two rows to the getAminoAcids() data.frame would have a detrimental impact on anything, so even if the gain of adding U and O is minimal, there's no real disadvantage.

@sgibb - what do you think? If you are OK, I suggest that @guideflandre sends a PR with the two extra lines/aa.

[sgibb]

Should be fine. Let's add them.