2 ideas for the next generation of data files

[mabruzzo]

Let me know if you would like me to move this over to the grackle-data-files repository.

I just wanted to share 2 quick ideas:

1. Related to PR Updating behavior of HydrogenFractionByMass parameter #200, I think we should attach an attribute, called "HydrogenFractionByMass", to the "CoolingRates/Primordial" hdf5 Group.

   • In that attribute, we would record the hydrogen-mass-fraction that Cloudy assumed
   • (I obviously don't care about the exact name/location of the attribute)
   • It may be worth modifying the existing tables to include this information

2. It could be cool to include a little more information to aide with post-processing of simulations of primoridal_chemistry=0 simulations

   • Context: @gregbryan and I have been involved in a project where we have been post-processing a number of primoridal_chemistry=0 simulations to try to study some mock observables. For these projects, we have used the mean molecular weight (and HydrogenFractionByMass) to back out the number density of free electrons. Then with some assumptions, we inferred a number density of neutral Hydrogen.
   • if it's not too much trouble, it would be extremely useful if the Grackle data-file could also provide grids of the:
     • fraction of Hydrogen that is ionized
     • fraction of Helium that is single-ionized
   • this would provide us with enough information to back out the fraction of Helium that is doubly ionized.
   • we could theoretically write pygrackle functionality to compute this derived information