Check that the initial condition setup for ionizing equilibrium works correctly.

GEAR-RT can set the initial ionization state of the gas to the (collisional) 
ionization equilibrium at startup. This test makes sure that this setup works as
expected. To do this, we set up particles in with a very high range of 
different internal energies, and only run for 1 step. The correct ion mass 
fractions should already be set in the zeroth output.

Compile swift with:
    --with-rt=GEAR_1 --with-rt-riemann-solver=GLF --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none 


Additional notes:

-   careful if running with periodicity on. The high difference in internal 
    energies across the box edge will induce a mass flux when running with
    meshless schemes, and the solution won't be exact at output_0000 any
    longer since the particles will be kicked before the output is written.