Looks for an added DataSetMerger derived from the given class.

    Args:
      cls: A class derived from DataSetMerger.

    Returns:
      The matching DataSetMerger instance.

    Raises:
      LookupError: No matching DataSetMerger has been added.

def GetMerger(self, cls):
    """Looks for an added DataSetMerger derived from the given class.

    Args:
      cls: A class derived from DataSetMerger.

    Returns:
      The matching DataSetMerger instance.

    Raises:
      LookupError: No matching DataSetMerger has been added.
    """
    for merger in self._mergers:
      if isinstance(merger, cls):
        return merger
    raise LookupError('No matching DataSetMerger found')