Small improvements in TimeStepAlgorithm

• Removed internal variable from IterationNumberBasedTimeStepping - the same information is stored in the TimeStep objects ts_previous and ts_current
• Since the information _previous_time_step_accepted is already transported within ts_previous and ts_current the interface to TimeStepAlgorithm::next() is simplified

1. Feature description was added to the changelog
2. Tests covering your feature were added? - The internal changes are tested by the existing tests.