LoopSolver/IterationStep getOutput interplay

IterationStep provides the getOutput() method which is used by LoopSolver to display additional information for each step. There is no information for the IterationStep though whether the header or actual iteration step output is requested. The only class that I found that reimplements the getOutput() method is in tnnmgstep.hh (module dune-tnnmg) using what I consider kind of a workaround.

I propose to change the interface for the output in one of the following ways:

1. use getOutput() and getOutputHeader() methods instead
2. use getOutput(int i) to pass the step (i=-1 for header)
3. (tnnmgstep-like:) let IterationStep have an outStream that any iterationStep can write to and let LoopSolver read from that instead.

Maybe I have overlooked something. If not, I would be willing to propose respective commits (also for dune-tnnmg). My favorite option is 1.