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/plot3dToFoam/plot3dToFoam.C

@@ -50,7 +50,6 @@ Description
 #include "wallPolyPatch.H"
 #include "symmetryPolyPatch.H"
 #include "cellShape.H"
-#include "cellModeller.H"
 #include "mergePoints.H"
 
 using namespace Foam;
@@ -202,7 +201,7 @@ int main(int argc, char *argv[])
 
     cellShapeList cellShapes(nMeshCells);
 
-    const cellModel& hex = *(cellModeller::lookup("hex"));
+    const cellModel& hex = cellModel::ref(cellModel::HEX);
 
     label nCreatedCells = 0;