Move parameter-value retrieval from EnzoPhysicsCosmology from EnzoConfig into constructor

[mabruzzo]

Pull request summary

Followup from #399. Moved parameter-value retrieval for EnzoPhysicsCosmology from EnzoConfig into constructor (and helper functions).

Detailed Description

This is part of the large migration of Parameter Parsing (that has been split into multiple parts to ease the entire process). The benefits of this migration are highlighted in the user-documentation on the website.

This PR touched slightly more files than other PRs of this general style. I needed to tweak a bunch of places that were directly accessing an EnzoConfig attribute called physics_cosmology. Now those locations just check whether enzo::cosmology()!=nullptr, instead (this is equivalent, we're checking if we're using cosmology). While I was doing that, I realized that some locations included I removed unnecessary accesses to EnzoConfig (most of these accesses became unnecessary because they no longer physics_cosmology, but a few were unnecessary before this change). All of these particular changes were concentrated into the first commit.

The second commit is where I actually relocated the parameter-retrieval logic.

[jwise77]

All looks good! Thanks for your patience in my review. Once you address the grackle conflicts, we can merge into main.