controlDict: Added optional 'beginTime' entry

for cases started or restarted from some arbitrary time without a corresponding
<time>/uniform/time dictionary to set the beginTime to the logical start of the
simulation.  Setting beginTime to 0 for example ensures that write times,
functionObject evaluations and writes etc. occur at intervals starting from 0
rather than the arbitrary startTime.

src/OpenFOAM/db/Time/Time.C

@@ -239,7 +239,11 @@ void Foam::Time::setControls()
         )
     );
 
-    if (timeDict.found("beginTime"))
+    if (controlDict_.found("beginTime"))
+    {
+        beginTime_ = userTimeToTime(controlDict_.lookup<scalar>("beginTime"));
+    }
+    else if (timeDict.found("beginTime"))
     {
         beginTime_ = userTimeToTime(timeDict.lookup<scalar>("beginTime"));
     }