Ion Mass Fraction Advection Test

In some schemes, like the meshless (gizmo) scheme, particles exchange 
masses via fluxes. This example tests whether the mass flux exchange 
is done correctly by setting up regions of special ionization states
and advecting them through time.

To use the GEAR RT model, compile with :
    --with-rt=GEAR_1 --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