Merge branch 'array-context' into 'master'

examples/advection/var-velocity.py

examples/advection/weak.py

examples/dagrt-fusion.py

examples/geometry.py

examples/maxwell/cavities.py

examples/wave/var-propagation-speed.py

examples/wave/wave-eager-mpi.py

examples/wave/wave-eager-var-velocity.py

examples/wave/wave-eager.py

examples/wave/wave-min-mpi.py

examples/wave/wave-min.py

grudge/discretization.py

grudge/eager.py

grudge/execution.py

grudge/function_registry.py

grudge/models/em.py

grudge/models/wave.py

grudge/shortcuts.py

grudge/symbolic/compiler.py

grudge/symbolic/operators.py