GEARRT cosmological thermochemistry

Goal: have thermochemistry work with cosmological runs

Implemented changes:

• Added comoving and physical getters for rt_radiation_energy_density
• Added CosmoHeatingTest and CosmoCoolingTest
• Added rt_props_update() function to update Grackle units
• Pass physical quantities to Grackle
• Fixed the exponent for internal energy in hydro_io

Note I have set up the code to convert to the physical frame for Grackle to solve thermochemistry in. In principle, Grackle has the utilities to solve in the comoving frame, too, but the results differ. This is an issue with Grackle that Mladen has raised with the Grackle people. With this implementation, rt_props_update() doesn't actually update the Grackle units (they are solving in the physical frame, after all), but I have left it in for if (when) updating becomes necessary