Subclasses / metadata for fasta sequence type (protein / nucleotide)

[bernt-matthias]

It could be handy to give the tools the possibility to find out id a fasta dataset is nucleotide / aminoacid.

• Add metadata
• Implement subtypes

The problem is not trivial/impossible if one tries to cover all corner cases, but I guess it should not be to hard to come up with a solution that works in most cases. For instance, check if a large majority of the characters are of the corresponding alphabet, e.g. 90%+x?

One could also just store the alphabet as metadata and leave the logic to the tool?