Recover initial time step after simulation end

This ensures that after every data assimilation step, the simulation
reverts to the original time step. Users may now use the setting for
`startTimestep` to optimize the time steps to the data assimilation
steps.

dune/dorie/common/time_controller.hh

@@ -32,6 +32,7 @@ private:
 	R tBegin; 				//!< simulation time limits
 	R tEnd; 				//!< simulation time limits
 	const int itmax, itmin; //!< newton iteration limits
+	const R dt_begin;		//!< beginning time step
 
 	const TSC& tsc; 		//!< Class with Function getNextTimeStamp()
 
@@ -61,6 +62,7 @@ public:
 		, itmax(config.get<int>("time.maxIterations"))
 		, itmin(config.get<int>("time.minIterations"))
 		, tsc(tsc_)
+		, dt_begin(dt)
 	{
 		if (not input_valid()) {
 			DUNE_THROW(IOError, "Invalid input for time controller");
@@ -134,11 +136,13 @@ public:
 
 			// Simulation has ended. Do nothing
 			if (time > tEnd) {
+				dt = dt_begin; // in case this was a data assimilation step
 				return true;
 			}
 			// NOTE: Simulation needs to know that we are actually at the end
 			else if (time <= tEnd && time >= tEnd-eps) {
 				time = tEnd;
+				dt = dt_begin; // in case this was a data assimilation step
 				return true;
 			}