* First cut of supporting quantiles ("percent ranks") per allele. Closes #87.
* Some refactoring of amino_acid.py

This builds on #114

* Switch from JSON to YAML for hyperparameters
* Pin tensorflow (hoping will help with #110)

This should give a substantial prediction speed improvement as identified by @rohanpai

* Add borrow_cached_network() method to Class1NeuralNetwork
* Use cached models at in Class1NeuralNetwork.predict
* Set names for all neural network layers so the model JSON is identical for identical architectures

* Add more experiments to models_class1_experiments
* Add `--quantitative-only` flag to training script
* Add `--no-throw` flag to prediction script