code: public DataStore getOrAddDataStore(final Collection<String> columns) {
          DataStore ret = getDataStore(columns);
          if (ret == null) {
              ret = this.createDataStore(columns);
          }
          return ret;
      }
docs: Get the DataStore associated with a collection of column names, adding a new one if
  necessary.
  
  @param columns Collection of columns to find a corresponding DataStore for.
  @return DataStore corresponding to the columns.