make a bunch of single arg ctors explicit

+ adjust test accordingly
[...block] updated grid layer usage
[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
update type_utils includes, closes #1
[discretefunction.default] let local_discrete_function return a unique_ptr

WARNING: this commit might brake your code!

refs #6
update local_discrete_function usage, closes #6
[test.spaces_dg_fem] disable polorder 2 tests in debug builds, refs #9
[spaces/test] there's no more Grid::ConstProviderInterface
fixes a few shadowing/unused/v.dtor. warnings