Fixes # 30. Conflicts:
	test/elliptic-sipdg-discretization.hh
	test/elliptic-swipdg-discretization.hh

This fixes #31. Conflicts:
	dune/gdt/discretefunction/default.hh

-instead of using Fem::GridWidth::calcGridWidth do calculation locally

- without it, compiler complains about pure virtual function

- without it, compiler complains about pure virtual function

These can be used to quickly prolong functions without the need to type
all the template arguments the compiler knows anyway:

* `GDT::Operators::prolong(target_grid_view, source, target);`
* `GDT::Operators::prolong(source, target);`

These can be used quickly obtain discrete functions instead of typing out
all the template arguments the compiler knows anyway:

* `auto const_func = GDT::make_const_discrete_function(space, vec);`
* `auto       func = GDT::make_discrete_function(space, vec);`
* `auto       func = GDT::make_discrete_function< VectorType >(space);`