Stop using DiscretizationCollection.normal in models and examples

examples/advection/surface.py

examples/geometry.py

examples/hello-grudge.py

examples/wave/wave-op-mpi.py

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

grudge/models/advection.py

grudge/models/em.py

grudge/models/euler.py

grudge/models/wave.py

test/test_grudge.py

test/test_op.py