Interpret [A, C, T, G, N] as dna_ext

[ErdaradunGaztea]

Currently the order of matching is dna_bsc -> rna_bsc -> ami_bsc -> dna_ext -> rna_ext -> ami_ext -> unt. However, [A, C, T, G, N] is matched by ami_bsc alphabet.

We should include this specific set of letters between rna_bsc and ami_bsc as dna_ext (and analogously for rna_ext).