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/advanced/splitCells/splitCells.C

@@ -52,7 +52,7 @@ Description
 #include "polyMesh.H"
 #include "cellCuts.H"
 #include "cellSet.H"
-#include "cellModeller.H"
+#include "cellModel.H"
 #include "meshCutter.H"
 #include "unitConversion.H"
 #include "geomCellLooper.H"
@@ -387,7 +387,7 @@ void collectCuts
     const vectorField& faceAreas = mesh.faceAreas();
 
     // Hex shape
-    const cellModel& hex = *(cellModeller::lookup("hex"));
+    const cellModel& hex = cellModel::ref(cellModel::HEX);
 
     // cut handling functions
     edgeVertex ev(mesh);