Merge branch 'mpi-communication' into 'master'

.gitignore

.gitlab-ci.yml

examples/benchmark_grudge/benchmark_mpi.py

examples/benchmark_grudge/run_benchmark.sh

examples/wave/wave-min-mpi.py

examples/wave/wave-min.py

examples/wave/wave.py

grudge/discretization.py

grudge/execution.py

grudge/symbolic/compiler.py

grudge/symbolic/dofdesc_inference.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

setup.py

test/test_mpi_communication.py