Morphology and amino acid support

[gaurav]

More or less works, but:

1. Exported datasets always come out as 'DNA'.
2. Check how symbolic exports work for TNT.
3. DNA ambiguity codes should be recoded as polymorphisms.
4. Allow for polymorphisms. DNA polymorphisms don't work at the moment, but we also need to support non-data polymorphisms.

See Nexus's "SYMBOLS" command to list out all the characters currently being used.