Merge branch 'external-call-v2' into 'master'

examples/gas_dynamics/lbm-simple.py

examples/wave/wiggly.py

grudge/execution.py

grudge/function_registry.py

grudge/models/em.py

grudge/models/gas_dynamics/__init__.py

grudge/models/wave.py

grudge/symbolic/compiler.py

grudge/symbolic/dofdesc_inference.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

test/test_grudge.py