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.

src/functionObjects/utilities/setTimeStep/setTimeStepFunctionObject.C

@@ -88,7 +88,7 @@ bool Foam::functionObjects::setTimeStepFunctionObject::execute()
     {
         const_cast<Time&>(time_).setDeltaT
         (
-            timeStepPtr_().value(time_.userTime())
+            timeStepPtr_().value(time_.userTimeValue())
         );
     }
 

src/functionObjects/utilities/setWriteInterval/setWriteIntervalFunctionObject.C

@@ -85,7 +85,7 @@ bool Foam::functionObjects::setWriteIntervalFunctionObject::execute()
 {
     const_cast<Time&>(time_).setWriteInterval
     (
-        writeIntervalPtr_().value(time_.userTime())
+        writeIntervalPtr_().value(time_.userTimeValue())
     );
 
     return true;