Pin keras version

Pin keras version. Also add note on KERAS_BACKEND to README.md.

Closes #67. Closes #65.

Kim2014 comparison, mhcflurry-predict tool, reimplement parallelization with futures

Fixes #54

Fixes #59

Update models_class1_allele_specific_single_kim2014_only to use hyperparameters roughly the same as preprint

Remove scripts for now

Tweak Dockerfile

Big refactor to prepare for release

Return the titin epitope tests back to the original more stringent threshold of 500nm (instead of 700nm)

Make test_imputation unit test fair by training the model with imputation for the same number of epochs as the model without imputation

Fix bug in model loading when the models dir is changes. Refactored into a Class1AlleleSpecificPredictorLoader class

Fix failing test test_class1_allele_specific_cv_and_train_command and switch downloads.yml to point to models without imputation for now