2D advection test for radiative transfer.

Test that your method is TVD and the propagation speed of the photons is
correct. The ICs set up four photon groups: 
- The first is a top hat function initial distribution where outside values
  are zero, advecting along the x direction
- The second is a top hat function initial distribution where outside values
  are nonzero. This distinction is important to test because photon energies 
  can't be negative, so these cases need to be tested individually. This
  group advects along the y direction
- the third is a smooth Gaussian advecting diagonally.
- the fourth is a circle in the center advecting radially.

This way, you can test multiple initial condition scenarios simultaneously. 
There are no stars to act as sources. Also make sure that you choose your
photon frequencies in a way that doesn't interact with gas!

The ICs are created to be compatible with GEAR_RT. Recommended configuration:

GEAR_RT (with gizmo-mfv solver):
    --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

GEAR_RT (with sphenix SPH solver):
    --with-rt=GEAR_3 --with-rt-riemann-solver=GLF --with-hydro-dimension=2 --with-hydro=sphenix  
     --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT

SPHM1RT:
    --with-rt=SPHM1RT_4 --with-hydro-dimension=2 --with-stars=basic   --with-sundials=$SUNDIALS_ROOT
    

IMPORTANT: Need SUNDIALS version  = 5 . 
SUNDIALS_ROOT is the root directory that contains the lib and include directories, e.g. on cosma:
SUNDIALS_ROOT=/cosma/local/sundials/5.1.0/

Note that in SPHM1RT any (SPH) hydro scheme is compatible.

Note that if you want to use a reduced speed of light for this test, you also 
need to adapt the fluxes in the initial conditions! They are generated assuming
that the speed of light is not reduced.