functionObjects::timeControl: Updated to use beginTime

for consistency with the handling of restarts in Time.  See

commit 7aca9bfa78
Author: Henry Weller <http://cfd.direct>
Date:   Fri Dec 11 13:37:16 2020 +0000

    Time: Added beginTime used to rationalise the write times when restarting

    Now if a case is restarted from an arbitrary time, for example one generated at
    a premature stop condition, or with an increased writeInterval, the subsequent
    time directories written are referenced to the original start time of the case
    rather than the restart time.

src/OpenFOAM/db/functionObjects/timeControl/timeControlFunctionObject.C

@@ -165,7 +165,7 @@ Foam::scalar Foam::functionObjects::timeControl::timeToNextWrite()
             (
                 0.0,
                 (writeTimeIndex + 1)*writeInterval
-              - (time_.value() - time_.startTime().value())
+              - (time_.value() - time_.beginTime().value())
             );
     }