Add RAII to ug domain creation/deletion in grid factory

The ugDomain (I don't know what exactly this is) seems to be created and destroyed without RAII. This MR at least fixes the memory leaks from #188 (closed) by using RAII when the first pointer is created.

Close #188 (closed)

added bugfix label

requested review from @oliver.sander

Closing in favor of !755 (merged)

closed