fvMesh: Split construction

This provides finer control as to when the changes get constructed and
initial stitching is performed. These is needed by certain processes,
notably decomposition and reconstruction.

By default, the mesh still performs all these operations on a "normal"
read-construction. A flag has to be passed explicitly to the constructor
in order to prevent the post-construction steps.

applications/utilities/postProcessing/graphics/PVReaders/vtkPVFoam/vtkPVFoam.C

@@ -477,10 +477,11 @@ void Foam::vtkPVFoam::updateFoamMesh()
                 dbPtr_(),
                 IOobject::MUST_READ
             ),
-            false,
-            fvMesh::stitchType::nonGeometric
+            false
         );
 
+        meshPtr_->postConstruct(false, fvMesh::stitchType::nonGeometric);
+
         meshChanged_ = true;
     }
     else