Add a group by feature

Reporting module works really well, however I am missing a group by feature.

I would like a seperate input where you can choose a specific variable/hostgroup or whatever to group by.
The filter should work like it does today, but if you set the group by argument the resulting report should look like this if you choose for example hostgroup:
hostgroup1: - total sla% for this group
host1 - sla%
host2 - sla%
hostgroup2: - total sla% for this group
host3 - sla%
host4 - sla%

The only alternative that i have come up with is one seperate report per desired group by. Which will result in like 30 reports for me that could have been one.

I'm unsure if this will cause issues when a host is part of the same hostgroup, i will do some exploring on my own as well.

No files, tests, or entry points are named. Start by locating the reporting module's filter and report-rendering paths, then determine how a selected hostgroup or variable could produce grouped totals and per-host results. Done should include grouped output while preserving current filtering, with behavior defined for hosts in multiple groups.