Remember it at the group so if for some reason the group is not
calculated as group then the individual cells of the group don't
iterate over the RPN again to calculate the same value.

This does not solve tdf#114251 in any way, just side-cars.

Change-Id: Ia26d4a7fe608d8aaa30686ce95c1caa25da02469
Reviewed-on: https://gerrit.libreoffice.org/46601Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>