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.

tutorials/incompressible/pimpleFoam/laminar/planarPoiseuille/validation/WatersKing/WatersKing.C

@@ -102,7 +102,7 @@ int main(int argc, char *argv[])
     const scalar LOGvGreat = ::log(vGreat);
     while (!runTime.end())
     {
-        scalar t = runTime.userTime()/lambda;
+        scalar t = runTime.userTimeValue()/lambda;
         forAll(A, i)
         {
             if (bk[i]*t < LOGvGreat)