Put sum factorization kernel implementations into functions

The amount of LOC in our generated operators will necessarily bring us into trouble at some point. Alternatively, each realization could spit out a loopy kernel (aka a method on the local operator) and the assembly method only sets up the input for that and calls it. Our current infrastructure with all the information being on a the SumfactKernel node is quite good for that: It can implement a hash, where hash equality means same function.