Iterative solver cleanup

This MR removes duplicate code in iterative solvers by moving

• convergence check
• invalid defect norm check (inf or NaN)
• output

to a class Iteration. A instance of this class is created with IterativeSolver::startIteration and is intended to be kept in the local scope of the apply method. During iteration the method Iteration::step checks for all the things mentioned above. The destructor fills all fields of InverseOperatorResult and prints the final statistics if verbose is positive.