Provide routine to return the step length taken

The output routine needs to know which step that has been taken in the
past (to arrive at this result), not which step to take next going
forward.

opm/simulators/timestepping/SimulatorTimer.cpp

@@ -89,6 +89,12 @@ namespace Opm
         return timesteps_[current_step_];
     }
 
+    double SimulatorTimer::stepLengthTaken() const
+    {
+        assert(current_step_ > 0);
+        return timesteps_[current_step_ - 1];
+    }
+
     /// Current time.
     double SimulatorTimer::currentTime() const
     {