Make save_model parametric in test_fn
Make save_model parametric in test_fn

kopt doesn't support model saving for models with custom activation
functions. So in these cases it's not plausible for users to test model
saving with test_fn. Let's make it optional by adding a save_model
argument similar to the one in CompileFN.

Also, users may not wish using test_fn with a test set i.e. a data_fn
that returns (x_train, y_train) tuple. Right nowt a second set is mandatory
in data_fn. I think we can relax this by just using the training set in test_fn.

Avsecz/keras-hyperopt