Count3mers and CodonFrequencyByAA added

[ewallace]

Hello,
I took inspiration from your CAI functions to:

• create a separate Count3mers function to count
• simplify the RSCU function to call Count3mers
• added CodonFrequencyByAA function to make a dict of counts by aa, which is useful for DNAChisel

I don't have tests yet and haven't checked edge cases, e.g. zero codon counts.

If this package is still being updated, let me know if any of these are interesting and what changes needed to incorporate.

I'm surprised that these sorts of functions aren't in core Biopython, still.

Best
Edward