Make it clearer that `default_simplex_group` is not for external use

By deprecating it and adding an underscored version.

By "definition", the function is not precise about what local discretization is chosen, which is a bad idea for code that expects consistent results run-to-run. It's currently used in mirgecom here:

https://github.com/illinois-ceesd/mirgecom/blob/0a43d11bdf5a21552a875636d8e5cefe23f76ab7/mirgecom/discretization.py#L59-L60=

and its use has also leaked into some grudge examples.

cc @MTCam @lukeolson