BUG: Change grouping from `vg_id` back to `hh_id`

### Bug description

In #662 we changed the grouping unit for Wohngeld from `hh_id` to `vg_id`. It turned out that this is not correct because having the same `vg_id` (being in one Verantwortungs- und Einstehensgemeinschaft) is a subset of the relevant grouping criteria for Wohngeld. The correct grouping is `hh_id`.