code: def load_sources(filename):
      
      catalog = catalogs.table_to_source_list(catalogs.load_table(filename))
      logging.info("read {0} sources from {1}".format(len(catalog), filename))
      return catalog
docs: Open a file, read contents, return a list of all the sources in that file.
      @param filename:
      @return: list of OutputSource objects