Metal advection test for the GEAR/AGORA chemistry schemes

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.

To run this test with GEAR, compile with:
    --with-hydro-dimension=2 --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-chemistry=GEAR_X
with X the desired number of elements. The number of elements must be also be set in the makeIC.py script
and must be the equal to one used in the compilation flag. The default value is 4.

To run this test with AGORA, compile with:
    --with-hydro-dimension=2 --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-chemistry=AGORA
NOTE: The AGORA scheme only traces Fe mass and total metal mass, so the number of elements in makeIC.py must be set
to the value of 2.

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.