Important optimization for pan-allele models: pass network the 'name' of the allele rather than the sequence. Network uses a fixed non-trianable allele encoding that maps allele 'names' to sequences.

Fix bug that led to NaN mhcflurry_prediction_high and mhcflurry_prediction_low uncertainty estimates in some cases

Performance enhancement for faster start time: reuse compiled networks even if their regularizations differ. This is safe because regularization should not affect predictions

Docs improvememnts, more flexible Keras dependency, drop typechecks dependency, remove some unused functions