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/OpenFOAM/meshes/treeBoundBox/treeBoundBoxTemplates.C

@@ -26,10 +26,8 @@ License
 #include "treeBoundBox.H"
 #include "FixedList.H"
 
-
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
-
 template<unsigned Size>
 Foam::treeBoundBox::treeBoundBox
 (