To add a new character frequency dataset using this API, a method just needs to be added to the
charfreq function. It should have the signature
function charfreq(cf::CustomDataSetStruct)::CharacterFrequency end
CustomDataSetStruct is any struct that you define for that particular character frequency data set. If needed, the constructor for the struct should take any arguments (e.g. see
SimplifiedLCMC for an example with arguments).
The return value should have type