Nn run on test generator
This merge addresses issues #116 (closed) and #117 (closed) by:
- Cleaning the duplicate run_on_test_generator method in the NNInterface class
- Adding the option to return a dictionary with metrics when calling the run_on_test_generator method
- Adding an assertion to verify that the checkpoint_freq does not exceed the number of epochs.
- Adding assertions to the NNInterface.save_model() method, which raise and error if no checkpoints are found