CRM-13265 made bi-directional toggle option available, keeping uni-direc...

...tional as default for core.

The reason it was uni directional because - stats reports like contribution summary initially, were also meant to work as detail report. And there could be cases where group-by could be optional.

The fix allows setting bi-directional option keeping uni-directional as default.
Also see https://github.com/civicrm/civihr/pull/117.