Filter out sequences that contain unknown amino acids

https://jira.gsc.wustl.edu/browse/AT-623
I dunno; it might be clearer in the future as it gets reused more.  Since `%convert` already has this special `Z` it seems like, theoretically, other special cases could be added which would affect your `is_valid_sequence`.  However, you couldn't really test `is_valid_sequence` without essentially duplicating the code since the code is primarily data.  Actually, yeah, moving `is_valid_sequence` might make sense in that it would make it clear that `%amino_acids` is being used for it.  You also wouldn't have to make `sub amino_acids` anymore.  Furthermore, if you export `is_valid_sequence` it would become more obvious that that functionality is intended for reuse.