Merge pull request #83 from inducer/thg/disc_tags

examples/advection/surface.py

examples/advection/var-velocity.py

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

grudge/discretization.py

grudge/dof_desc.py

grudge/execution.py

grudge/models/advection.py

grudge/op.py

grudge/symbolic/dofdesc_inference.py

grudge/symbolic/mappers/__init__.py

grudge/symbolic/operators.py

grudge/symbolic/primitives.py

test/test_grudge.py

test/test_modal_connections.py