add pylint and fix issues

.github/workflows/ci.yml

.gitlab-ci.yml

.pylintrc-local.yml

examples/dagrt-fusion.py

grudge/discretization.py

grudge/eager.py

grudge/models/__init__.py

grudge/models/em.py

grudge/models/gas_dynamics/__init__.py

grudge/models/wave.py

grudge/op.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/tools.py

test/mesh_data.py

test/test_grudge.py