Redesign operator language to capture fluxes, include DOFDesc

examples/wave/wave-min.py

grudge/__init__.py

grudge/discretization.py

grudge/execution.py

grudge/models/wave.py

grudge/symbolic/compiler.py

grudge/symbolic/flux/__init__.py

grudge/symbolic/flux/mappers.py

grudge/symbolic/flux/primitives.py

grudge/symbolic/flux/tools.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

grudge/symbolic/tools.py

test/test_grudge.py