ENH: allow delayed construction of regionFaModel in volume bcs (#3419)

Ideally wish to delay construction of the finite-area mesh until
  updateCoeffs(), when it is actually needed.

  This helps avoid difficult to trace errors and avoids inadvertent
  parallel communication during construction from a dictionary.

  However, lazy evaluation fails at the later stage while attempting
  to load the corresponding initial fields, since the timeIndex has
  already advanced.

  Use the newly introduced regionModels::allowFaModels() switch
  to locally enable or disable lazy evaluation as needed.

  This allows selective disabling (eg, in post-processing utilities)
  where loading the volume field should not activate the associated
  regionFaModel and loading a finite-area mesh too (which may not
  exist or be defined at that point).

applications/utilities/postProcessing/dataConversion/foamToEnsight/foamToEnsight.C

@@ -323,6 +323,9 @@ int main(int argc, char *argv[])
     );
     argList::addOptionCompat("cellZones", {"cellZone", 1912});
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
+
     #include "setRootCase.H"
 
     // ------------------------------------------------------------------------

applications/utilities/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -479,8 +479,14 @@ int main(int argc, char *argv[])
         "Directory name for VTK output (default: 'VTK')"
     );
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
+
     #include "setRootCase.H"
 
+    // ------------------------------------------------------------------------
+    // Configuration
+
     /// const int optVerbose = args.verbose();
     const bool decomposePoly = args.found("poly-decomp");
     const bool doBoundary    = !args.found("no-boundary");