Metallicity Dependent Cooling and metal_density scalar field

[hannahjleary]

Summary

Adds metallicity-dependent cooling and a new scalar metal_density field to track metal content per cell.

Changes

-Added metal_density as a new tracked scalar field (using SCALAR and METALS macros). This is used by CoolRecipeMetals to compute the metallicity of a cell as metal_mass_frac / solar_metal_mass_frac.
-CoolRecipeMetals: Combines primordial cooling with metal-line cooling by scaling the metal-line contribution of a solar abundance Cloudy model by the cell's metallicity.
-CoolRecipePrimordial: added a recipe to use cool_component::primordial_cool. Requires heating to be disabled by setting a new parameter chemistry.enable_heating to false (default: true)
-All cooling recipes now take a hydrogen and helium number density (n_H and n_He, respectively), as well as a metallicity.
-Added documentation for the two new cooling recipe options, the METALS macro, and the metal_density output fields
-docs/sphinx/Physics/metallicities.png shows the cooling rates of CoolRecipeMetals for a range of metallicities.
-docs/sphinx/Physics/models.png shows the primordial cooling curve, the CoolRecipeMetals curve, and an actual Cloudy model for four different metallicities. Our model closely follows the true Cloudy run even in the low and high metallicity limits.
-The initial conditions function Clouds() now takes two new input parameters from pmap: metallicity_wind and metallicity_cloud (default value: 1.0)
-The initial conditions function Constant() now takes one new input parameter from pmap: metallicity (default value: 1.0)