Fix mhctools API integration

[tavinathanson]

From @iskandr re openvax/mhctools#86:

Followup to mhctools PR, a much smaller change to make Topiary work: openvax/topiary#66

The only API changes that should matter to you are epitope_lengths -> default_peptide_lengths in the initializer, predict -> predict_subsequences, addition of a predict_peptides if you need it, and the result of predict methods is just a list of BindingPrediction objects (not a special collection object)

And if you need all these things in data frames there’s predict_subsequences_dataframe and predict_peptides_dataframe

(instead of previoously converting a EpitopeCollection to a dataframe)