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

src/fileFormats/vtk/read/vtkUnstructuredReaderTemplates.C

@@ -27,7 +27,7 @@ License
 #include "labelIOField.H"
 #include "scalarIOField.H"
 #include "stringIOList.H"
-#include "cellModeller.H"
+#include "cellModel.H"
 #include "vectorIOField.H"
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //