Units Sub-Groups

hhamedk, you are right that it was requested a lot of times. But we found that we cannot implement them. Briefly, we have some variants:
- delete all current unit groups to all users and create a new system with subgroups.
- create something that will look like subgroups in the monitoring tab, but it won't work as a normal group in notifications, reports and so on
- leave the current structure
The third variant was chosen.