Vectorize local_grad and weak_local_grad (#87)

* 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>