public static Map<String, List<String>> parseGrouped(final String featureDir) throws IOException {
        return parseGrouped(featureDir, Collections.emptyList());
    }
Parses features to a {@code Map} structure of the test name as the key with a {@code List} of Gremlin strings
for the value.

@param featureDir The root directory where feature files can be found including subdirectories
gremlin-language/src/main/java/org/apache/tinkerpop/gremlin/language/corpus/FeatureReader.java