Cleanup/small

See merge request !87

Feature/symbolic parameter functions

MR for #37 

Just saving this here for backup and visiblity, totally WIP.

See merge request !57

which reuses the poisson_dg ufl file with variants

The old formula contained some 'dim's instead of '2's,
which obviously works for 2D....

Tricky... some grad components depended on the sumfac kernels of the
other directions instead of their input...

... really elegant.

Need to use handler from pymbolic as loopy codegen handlers
are too involved for standalone usage.

until poisson_neumann passes

FastDGGridOperator in driver

See merge request !84