Add --top-groups to rbh-report

[karcaw]

It would be very helpful to have a list of the top group usage, just like the --top-users option in rbh-report. it looks like one could just replicate the report_topusers function, and change the GROUP_BY setting to gid instead of uid. Is this the best way to do this? If so i could probably build up a Pull Request with that done.

[dtcray]

Yes writing top-groups as clone of top-users should work. for code contribution please read https://github.com/cea-hpc/robinhood/wiki/Contributing