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/postProcessing/dataConversion/foamDataToFluent/foamDataToFluent.C

@@ -55,7 +55,7 @@ int main(int argc, char *argv[])
     {
         runTime.setTime(timeDirs[timeI], timeI);
 
-        Info<< "Time = " << runTime.timeName() << endl;
+        Info<< "Time = " << runTime.userTimeName() << endl;
 
         if (mesh.readUpdate())
         {