Implemented interface method preprocess() (inherited from Solver).

Moved call of iterationStep->preprocess from LoopSolver::solve() to LoopSolver::preprocess()
ATTENTION:
This might affect your code although it's not an interface change. So far the LoopSolver::preprocess() method was empty and therefore it was working - although against the dune-solvers paradigm -
to call LoopSolver::solve() without calling LoopSolver::preprocess() before.
Now, YOU NEED TO CALL preprocess() otherwise your iteration step will not be initialized properly.

[[Imported from SVN: r7334]]