Non-fastdg: Permutation of the input happens before the sum factorization
kernel when we setup the input. This is done by a method of the corresponding
interface class.

Fastdg: In this case the input will always be ordered according to x,y,... This
means the permutation needs to happen in the sumfact kernel. Since we want to
vectorize sumfact kernels with different input permutation in an upper/lower
way we need to do this permutation in the corresponding interface class. This
is done in the realize_direct method and in the vectorized case the
corresponding methods of the scalar sumfact kernels are called.