Clearly documenting "radiation redshift" when comoving_coordinates == 0

[mabruzzo]

In Grackle, the redshift (specified as a_value) plays 2 distinct roles:

1. It always used to model the impact of background radiation. Concrete examples include:
   • the CMB temperature-floor.
   • Compton cooling off of the CMB.
   • treatment of the UV background. For primordial_chemistry > 0, the reaction rates are impacted. For primordial_chemistry == 0, both the tabulated heating/cooling rates and tabulated temperature calculations are impacted.
2. When comoving_coordinates == 1, it’s used to parameterize the conversion factor between a simulation's internal code-units and cgs-units changes over cosmic time.

The narrative documentation is currently not very clear about whether Grackle supports modifying a_value when comoving_coordinates == 0 after Grackle is initialized. This is important since Grackle’s comoving-unit-support is not currently compatible with most comoving coordinate systems.

In fact, I had assumed that Grackle didn't support changing the a_value. But, after looking at the code-base, I now think it does. I think we should:

1. Make this more clear in the narrative documentation
2. Maybe add a Pygrackle integration test that explicitly confirms that this behavior works as expected.