Chemistry API changes for metal fluxes

This merge request extends the chemistry API as discussed in #834 to correctly advect metals in hydro schemes with mass fluxes between particles.

The approach is very similar to @mivkov's approach to tracking the mass fractions of the ionizing species in GEAR-RT.

Current status

This is a working implementation for the EAGLE chemistry scheme. Chemistry schemes other than EAGLE and none currently throw a compile time error trying to compile with a hydro scheme that does mass fluxes (in this branch, only gizmo-mfv).

To-do

• Implement the new functions for other chemistry schemes
• How to treat diffusion in this case? (only necessary when implementing for GEAR-diffusion)
• Advect other quantities tracked by chemistry as well (e.g. mass_from_SNIa, metal_mass_fraction_from_SNIa and others)?

Other thoughts

As discussed in #834, we would also need to disable metal smoothing when using this with e.g. moving mesh hydro.