functionObject: Fixed bugs in function object time-step adjustment

The logic governing function objects' ability to change the time-step
has been modified so that it is compatible with the time-step adjustment
done in the Time class. The behaviour has been split into a method which
sets the step directly, and another which moidifies the time until the
next write operation (i.e., the time that the solver "aims" for).

This fixes an issue where the adjustments in Time and the function
objects interfere and cause the time step to decrease exponentially down
to machine precision. It also means that the set-time-step function
object now does not break the adjustable run-time setting.

This resolves bug report https://bugs.openfoam.org/view.php?id=2820

src/OpenFOAM/db/functionObjects/functionObjectList/functionObjectList.H

@@ -274,8 +274,11 @@ public:
         //- Called when Time::run() determines that the time-loop exits
         bool end();
 
-        //- Called at the end of Time::adjustDeltaT() if adjustTime is true
-        bool adjustTimeStep();
+        //- Override the time-step value
+        bool setTimeStep();
+
+        //- Return the time to the next write
+        scalar timeToNextWrite();
 
         //- Update for changes of mesh
         void updateMesh(const mapPolyMesh& mpm);