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/Time/Time.C

@@ -30,6 +30,7 @@ License
 #include "profiling.H"
 #include "demandDrivenData.H"
 #include "IOdictionary.H"
+#include "registerSwitch.H"
 
 #include <sstream>
 
@@ -77,6 +78,19 @@ const int Foam::Time::maxPrecision_(3 - log10(SMALL));
 
 Foam::word Foam::Time::controlDictName("controlDict");
 
+int Foam::Time::printExecutionFormat_
+(
+    Foam::debug::infoSwitch("printExecutionFormat", 0)
+);
+
+
+registerInfoSwitch
+(
+    "printExecutionFormat",
+    int,
+    Foam::Time::printExecutionFormat_
+);
+
 
 // * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
 
@@ -1161,7 +1175,7 @@ Foam::Time& Foam::Time::operator++()
             {
                 const label writeIndex = label
                 (
-                    returnReduce(label(elapsedClockTime()), maxOp<label>())
+                    returnReduce(elapsedClockTime(), maxOp<double>())
                   / writeInterval_
                 );
                 if (writeIndex > writeTimeIndex_)