exploit clusteredness of groups BAT to speed-up sub-grouping
exploit clusteredness of groups BAT to speed-up sub-grouping

In case the BAT with existing gropus is clustered
(i.e., sorted || revsorted), we can abandon the hash-loop
to search for equal (value,group-id) pairs as soon as we
leave the current group.
For this, we do exploit the fact that our hash-tables
link backwards through the BAT.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb