Fixed a bug in IterationNumberBasedTimeStepping

In the iteration number based time stepping, the next time step size is computed by multiplying the current time step size with a nonlinear iteration number related factor.

In the implementation of this time stepper, the current time step has to be assigned to previous time step, which is obviously the previous step to the next time step, for the computation of the next time step size. Unfortunately, in the existing implementation previous time step is not correctly set if a time step is rejected. This MR fixes this issue.

1. Feature description was added to the changelog
2. Tests covering your feature were added? use the existing benchmarks
3. Any new feature or behavior change was documented?