ENH: enumerations for known cell models in cellModel, ptr/ref lookups

- this provides a better typesafe means of locating predefined cell
  models than relying on strings. The lookup is now ptr() or ref()
  directly. The lookup functions behave like on-demand singletons when
  loading "etc/cellModels".

  Functionality is now located entirely in cellModel but a forwarding
  version of cellModeller is provided for API (but not ABI) compatibility
  with older existing user code.

STYLE: use constexpr for cellMatcher constants

applications/utilities/mesh/conversion/fluentMeshToFoam/extrudedTriangleCellShape.C

@@ -28,7 +28,7 @@ Description
 
 #include "cellShapeRecognition.H"
 #include "labelList.H"
-#include "cellModeller.H"
+#include "cellModel.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -52,7 +52,7 @@ cellShape extrudedTriangleCellShape
 
     if (!prismModelPtr_)
     {
-        prismModelPtr_ = cellModeller::lookup("prism");
+        prismModelPtr_ = cellModel::ptr(cellModel::PRISM);
     }
 
     const cellModel& prism = *prismModelPtr_;