ENH: make format of ExecutionTime = ... output configurable (issue #788)

- controlled by the the 'printExecutionFormat' InfoSwitch in
  etc/controlDict

      // Style for "ExecutionTime = " output
      // - 0 = seconds (with trailing 's')
      // - 1 = day-hh:mm:ss

   ExecutionTime = 112135.2 s  ClockTime = 113017 s

   ExecutionTime = 1-07:08:55.20  ClockTime = 1-07:23:37

- Callable via the new Time::printExecutionTime() method,
  which also helps to reduce clutter in the applications.
  Eg,

     runTime.printExecutionTime(Info);

  vs

     Info<< "ExecutionTime = " << runTime.elapsedCpuTime() << " s"
         << "  ClockTime = " << runTime.elapsedClockTime() << " s"
         << nl << endl;

--

ENH: return elapsedClockTime() and clockTimeIncrement as double

- previously returned as time_t, which is less portable.

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

@@ -217,7 +217,7 @@ bool Foam::timeControl::execute()
         {
             label executionIndex = label
             (
-                returnReduce(label(time_.elapsedClockTime()), maxOp<label>())
+                returnReduce(time_.elapsedClockTime(), maxOp<double>())
                /interval_
             );
             if (executionIndex > executionIndex_)