Remove uses of deprecated actx.traversal.{freeze,thaw}

examples/advection/surface.py

examples/advection/var-velocity.py

examples/advection/weak.py

examples/euler/acoustic_pulse.py

examples/euler/vortex.py

examples/geometry.py

examples/hello-grudge.py

examples/maxwell/cavities.py

examples/wave/var-propagation-speed.py

examples/wave/wave-min-mpi.py

examples/wave/wave-op-mpi.py

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

grudge/array_context.py

grudge/discretization.py

grudge/dt_utils.py

grudge/geometry/metrics.py

grudge/models/advection.py

grudge/models/em.py

grudge/models/euler.py

grudge/models/wave.py