[NumLib/TimeStepping] Small fix on IterationNumberBasedTimeStepping

Introduced information flow whether the timestep was accepted or not.

This prevents the times in the timestepper and the external loop to diverge
which causes two issues otherwise:
1.) Endless loop at t --> t_end, if max_iter > largest multiplier
2.) Endless loop at t --> t_end, if nonlinear solver fails during
	execution