Trainer and FeatureExtractor updates

Changed non-numeric or identifying feature names to have '_' prefix for
easy filtering.
Moved FeatureExtractor to new extractor.py.
Added sort_headers to get features in specified order.
Added convert_csv_for_probtype to split big csv file with identifiers
into BtL and WtK files with only numeric features + binary outcome.
Removed PatchExtractor from list of features to extract.