Improve backtranslation functionality.
Now, it considers the presence of:
 .- Selenocysteines ('U') coded by the stop-codon 'TGA'.
 .- Pyrrolysines ('O') coded by the stop-codon 'TAG'.
 .- Presence of wildcards/indetermination characters such as 'B' or 'X' at amino-acidic sequence level.