Metal advection test for the EAGLE chemistry scheme

In some schemes, like the meshless (gizmo) scheme, particles exchange 
masses via fluxes. This example tests whether the metals are correctly
advected with those mass fluxes (for the EAGLE chemistry scheme)

To run this test, compile with:
    --with-hydro-dimension=2 --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-chemistry=EAGLE

Due to the nature of this test, not much mass will be exchanged when running in Lagrangian mode,
and hence not much advection will happen.
To be able to see the effect of the advection, the hydrodynamics must be run in Eulerian mode.
E.g. for gizmo-mvf: uncomment `#define GIZMO_FIX_PARTICLES` in src/const.h.

Expected output when running in Eulerian mode is that the profiles should maintain their shape,
but edges will be blurred due to numerical diffusion.