add maxSteps to {Writer,ParallelWriter,Steerer}::stepFinished()

[gentryx]

It's a source for nasty errors that Writers and Steerers need to save their own copy of maxSteps, as the user may easily forget to initialize them correctly. This change would remove such duplication by forwarding the data directly from the Simulator.

Downside: all Steppers need to know it, too.

Moved from https://bitbucket.org/gentryx/libgeodecomp/issues/11/add-maxsteps-to-writer-parallelwriter