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/solvers/combustion/chemFoam/chemFoam.C

@@ -42,7 +42,7 @@ Description
 #include "OFstream.H"
 #include "thermoPhysicsTypes.H"
 #include "basicMultiComponentMixture.H"
-#include "cellModeller.H"
+#include "cellModel.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 

applications/solvers/combustion/chemFoam/createSingleCellMesh.H

@@ -13,8 +13,7 @@ points[5] = vector(1, 0, 1);
 points[6] = vector(1, 1, 1);
 points[7] = vector(0, 1, 1);
 
-const cellModel& hexa = *(cellModeller::lookup("hex"));
-faceList faces = hexa.modelFaces();
+faceList faces = cellModel::ref(cellModel::HEX).modelFaces();
 
 fvMesh mesh
 (
@@ -25,7 +24,7 @@ fvMesh mesh
         runTime,
         IOobject::READ_IF_PRESENT
     ),
-    xferMove<Field<vector>>(points),
+    xferMove<pointField>(points),
     faces.xfer(),
     owner.xfer(),
     neighbour.xfer()