amino acid alphabet

[tijeco]

This is definitely only a minor thing, but I think is worth addressing. I personally only really care about the standard 20 amino acids, so I have no interest in predictions for other amino acids that these models may have support for. For now, I have hard coded the 20 amino acids as a string.

This creates two issues. First, others may be interested in other amino acids that are supported by these models. Second, even if that's not the case, I think it would be better to have a centralized definition of the alphabet instead of hard coding it numerous times in other functions.

What I think would make the most sense is to maybe make a utils module or core module and put things like this here so that other modules just import the alphabet.