Returns true if a file is required by GTFS, false otherwise.
    Unknown files are, by definition, not required

def IsFileRequired(self, filename):
    """Returns true if a file is required by GTFS, false otherwise.
    Unknown files are, by definition, not required"""
    if filename not in self._file_mapping:
      return False
    mapping = self._file_mapping[filename]
    return mapping['required']