Collide two photon packets with each other, and see what happens.
Two photon groups contain packets using a top hat functions. One of
them has the lower value at zero, the other is at nonzero. The third
photon group packets are Gaussians.
Ideally, they should just pass through each other. But that is not
what moment based methods do with radiation.


To run with GEAR-RT, compile SWIFT with

    --with-rt=GEAR_2 --with-rt-riemann-solver=GLF --with-hydro-dimension=1 --with-hydro=gizmo-mfv \
     --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT

