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_SNIaand others)? 
Other thoughts
As discussed in #834, we would also need to disable metal smoothing when using this with e.g. moving mesh hydro.
Edited  by Yolan Uyttenhove