- a valid() method (same as !empty() call) for consistency with other
containers and data types
- a centre() method (same as midpoint() method) for consistency with
other OpenFOAM geometric entities
- 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
