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)