Chemistry: Metal fluxes for GEAR and AGORA (and QLA)

I also implemented the metal advection for the GEAR and AGORA schemes (AGORA seems to just be a specific variant of GEAR?). See also #834 and !1825 (merged).

The QLA chemistry scheme looks at first glance like it is just a rename of the none chemistry scheme? So, I added the same no-ops for the metal advection, so that we can also compile with gizmo-mfv and QLA chemistry.