No warning is raised if ablation flag is used with train_file/test_file

[mulhod]

A warning should be raised if a user has specified a train_file/test_file in their config file and then tries to use ablation when running run_experiment. No featuresets are specified, so there's nothing to ablate. No errors occur and the experiments seem to finish fine, but the results don't make much sense in terms of the labels that are given to them. Probably a warning should be raised and the use of the ablation flag ignored OR an exception should be raised since it's unclear what the user is trying to do. Perhaps mention could be made that the user will have to specify a train_directory/test_directory in order to specify featuresets and/or that, in order to be able to ablate with each feature in a file, a different file will (currently) have to be made for each individual feature and specified as a featureset separately. (This last issued is related to another issue and the behavior might change if dealt with.)

[desilinguist]

Addressed by #392.