The realize_output interface method was not really necessary and just adds a
layer of indiretion.

Due to a bug in dune-geomtry the volume calculation is not accurate enough. For
now we just remove all volume calls from the weak formulation.

Can me generated by calling generate_meshfiles.py

Have to investigate further. The change in the residual seems way too
large. The curious thing is that the input to the sum factorization kernels
looks good and the poisson without sum factorization gives the same
solution. Very unexpected.

There was a minus sign error...

Instead of generating gmsh files for a structured grid we use a real
unstructured grid.

The residuals of these tests should be the same modulo permutation and fuzzy
float compare.

Introduce different methods for realize_input/output
realize_direct_input/output and setup_input/output. The setup methods cover
code generation outside the sumfact kernel function (creating input array or
accumulating result). realize and realize_direct handle the input/output in the
nonfastdg and fastdg code branch.

Seperate interface methods make it a lot easier to find out where each of those
methods will be applied. Besides that most interface classes need to provide
more that two of those methods anyway...

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.

So far only for fastdg. This should also happen in the non-fastdg case.

Note: They are not yet used but in the long term the permutation should be
handled here since it is about input/output setup.