Merge branch 'array-context-merge' into 'array-context'

examples/dagrt-fusion.py

examples/wave/wave-eager-mpi.py

examples/wave/wave-eager.py

grudge/eager.py

grudge/execution.py

grudge/models/advection.py

grudge/models/em.py

grudge/models/wave.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

test/test_grudge.py

test/test_mpi_communication.py