- Rename, document the supporting functions for improved clarity
- Drop dim argument from (former) surface_normal

The function registry bits are only relevant for symbolic operators,
and the broadcasting test duplicates what's already being done in
arraycontext.

This ensures that the symbolic stuff remains tested until we remove it.

* vectorize local_grad and weak_local_grad

* added stack argument to local_grad/weak_local_grad

* add test for gradient using op

* propagate stack argument

* tweak comment to avoid flake8 warning

* stack -> (not) nested

* add visualization to gradient test

* disable strong form gradient test for now

* minor cosmetic changes

* tweak local_div/weak_local_div vectorization to work with both nested and non-nested arrays

* suppress pylint false positives

* remove inverse_mass around grad/div in strong form tests

* add check for valid 'form' argument

* remove unneeded flux terms from strong form tests

* Tweak phrasing: "no jumps" instead of "smooth"

Co-authored-by: Andreas Klöckner <inform@tiker.net>