[spaces.rt.pdelab] update copy behaviour, refs #28

I also removed the const of the `grid_view_` member to get rid of the
cast in the `backend_` ctor call. Is this preferable over the const_cast,
@renemilk?
[test.elliptic-...dg-discretization] disabled polorder 2 in debug mode

refs #26
replaced DS_OVERRIDE and DS_FINAL by override and final, respectively