[newchem-cpp] Remove Hard-coded injection path names from grackle_field_data

[mabruzzo]

To be reviewed after #479 is merged

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This PR replaces all hard-coded injection pathway data-members from the grackle_field_data (e.g. grackle_field_data::local_ISM_metal_density, grackle_field_data::y19_metal_density, ...) with an array of pointers known as grackle_field_data::inject_pathway_metal_density. In other words, Grackle no longer hardcodes the names of each injection pathway members as part of the grackle_field_data struct. (To determine the names, users must use the query-machinery introduced in #479).

This is significant for 2 reasons:

1. I think the old approach would have been a pretty bad choice from an API perspective
   • injection pathway models or supernova models is the exact sort of thing where one might expect "churn"
   • under the old approach, we would have needed to keep adding more and more members to grackle_field_data whenever someone wanted another model (while maintaining members for all existing models)
   • Likewise, a simulation code that wanted to use another model would need to add several lines of code to support each kind of model
   • the newer approach is much more scalable
2. It's also important from an implementation perspective:
   • the number of inject field paths are now entirely determined by the values read in from the table. To add/remove/rename pathways, you "just" need to modify inject_model/raw_data.[ch]pp and inject_model/load_data.[ch]pp.
   • this is a massive step that will make it possible to move this data into HDF5 tables (and let users supply arbitrary data)