Feature Request: Custom Codon Tables

[aubreybailey]

Great work folks!
I was using dnachisel for non-model organisms, and wondered if it would be reasonable to allow the substitution of a custom codon frequency table from an uploaded JSON or CSV.

I think I can volunteer a snippet to convert a pandas df to the nested dictionary format that dnachisel wants.

potential impact: this might cause interface logic problems with your new codon-pair additions.