Protein-related functions

[jlboat]

I primarily work with nucleotide sequences in DNA format, but am considering expanding functionality to proteins. However, I also want to maintain simplicity as this is why this package was designed in the first place.

[jlboat]

Converted Sequence to abstract and added DNA and RNA which extend Sequence as a first step toward this goal. Sequence functions are also protein applicable. However, accessing DNA/RNA/Protein functions looks like it will require casting. Not ideal, but seems to work fine so far.