Time: Added run-time selectable userTime option

replacing the virtual functions overridden in engineTime.

Now the userTime conversion function in Time is specified in system/controlDict
such that the solver as well as all pre- and post-processing tools also operate
correctly with the chosen user-time.

For example the user-time and rpm in the tutorials/combustion/XiEngineFoam/kivaTest case are
now specified in system/controlDict:

userTime
{
    type     engine;
    rpm      1500;
}

The default specification is real-time:

userTime
{
    type     real;
}

but this entry can be omitted as the real-time class is instantiated
automatically if the userTime entry is not present in system/controlDict.

applications/utilities/miscellaneous/patchSummary/patchSummary.C

@@ -69,7 +69,7 @@ int main(int argc, char *argv[])
     {
         runTime.setTime(timeDirs[timeI], timeI);
 
-        Info<< "Time = " << runTime.timeName() << nl << endl;
+        Info<< "Time = " << runTime.userTimeName() << nl << endl;
 
         // Update the mesh if changed
         if (mesh.readUpdate() == polyMesh::TOPO_PATCH_CHANGE)