Fixes #693 (closed).
Several reference solutions (dumux and dumux-lecture) have to be updates because this change might change some time step sizes after check points significantly which can lead to slightly different results.
suggests the time step size before the last time step before a check point if the time step size had to be reduced to reach that check point
increases the time step suggestion before a check point by up to 20% if the check point can be reached exactly
makes floating point comparisons more robust by removing the dependence on the current simulation time