adds create_crossval_idx
adds create_crossval_idx

a helper function that creates the strings necessary to effectively
crossvalidate

also adds model_learning_hyperparams_full_cv, which does the same as
model_learning_hyperparams_full, but with leave-4-out cv

billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences,billbrod/spatial-frequency-preferences