Collide packets of radiation against each other, and see how the M1
closure treats them.

Collide them horizontally, vertically, diagonally, and from the side.

To run with GEAR-RT, configure SWIFT using

    --with-rt=GEAR_4 --with-rt-riemann-solver=GLF --with-hydro-dimension=2 --with-hydro=gizmo-mfv \
     --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT