group: implemented co-group
group: implemented co-group

co-group can be considered to be a join, followed by a single-input
group.  The co-group is implemented as such 2-stage operation.

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