Clean up examples and various imports

examples/advection/surface.py

examples/advection/var-velocity.py

examples/advection/weak.py

examples/geometry.py

examples/maxwell/cavities.py

examples/old_symbolics/dagrt-fusion.py

examples/wave/var-propagation-speed.py

examples/wave/wave-min-mpi.py

examples/wave/wave-min.py

examples/wave/wave-op-mpi.py

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

examples/wave/wave-op.py

grudge/discretization.py

grudge/execution.py