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

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...