Generalized the fixed vector encoding length and moved around some components for testability

expose pretrain decay factor function

small fixes to random negative samples

Perform imputation in training script

Added unit for training model with imputed data

adding test to make sure imputation improves predictive accuracy on a small allele, added allow_unknown_amino_acids to from_hyperparameters

fixed neural network test, fixed length filtering of data loading, started on adding imputation to training script

AlleleData objects now have numpy arrays for all attributes