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/parallelProcessing/decomposePar/decomposePar.C

@@ -418,8 +418,14 @@ int main(int argc, char *argv[])
     // Allow explicit -constant, have zero from time range
     timeSelector::addOptions(true, false);  // constant(true), zero(false)
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
+
     #include "setRootCase.H"
 
+    // ------------------------------------------------------------------------
+    // Configuration
+
     const bool writeCellDist    = args.found("cellDist");
 
     // Most of these are ignored for dry-run (not triggered anywhere)

applications/utilities/parallelProcessing/reconstructPar/reconstructPar.C

@@ -135,9 +135,14 @@ int main(int argc, char *argv[])
         "Only reconstruct new times (i.e. that do not exist already)"
     );
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
+
     #include "setRootCase.H"
     #include "createTime.H"
 
+    // ------------------------------------------------------------------------
+    // Configuration
 
     const bool doFields = !args.found("no-fields");
     wordRes selectedFields;

applications/utilities/parallelProcessing/reconstructParMesh/reconstructParMesh.C

@@ -780,11 +780,17 @@ int main(int argc, char *argv[])
 
     #include "addAllRegionOptions.H"
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
+
     #include "setRootCase.H"
     #include "createTime.H"
 
     printWarning();
 
+    // ------------------------------------------------------------------------
+    // Configuration
+
     const bool fullMatch = args.found("fullMatch");
     const bool procMatch = args.found("procMatch");
     const bool writeCellDist = args.found("cellDist");

applications/utilities/parallelProcessing/redistributePar/redistributePar.C

@@ -1257,6 +1257,8 @@ int main(int argc, char *argv[])
         true  // Advanced option
     );
 
+    // Prevent volume BCs from triggering finite-area
+    regionModels::allowFaModels(false);
 
     //- Disable caching of times/processor dirs etc. Cause massive parallel
     //  problems when e.g decomposing.
@@ -1269,6 +1271,7 @@ int main(int argc, char *argv[])
 
     argList args(argc, argv);
 
+    // ------------------------------------------------------------------------
 
     // As much as possible avoid synchronised operation. To be looked at more
     // closely for the three scenarios: