RT GEAR thermochemistry first attempt

This MR introduces the first working version for GEAR-RT including thermochemistry. This includes:

• doing cooling, heating, and ionizing with grackle during the thermochemistry tasks, and modifying the radiation fields appropriately
• added option to choose stellar spectra, which are used to estimate average interaction cross sections over a photon frequency interval. Currently have constant and blackbody spectrum ready.
• added flux exchange of ionizing species during hydro
• added call to rt_kick_extra, where extra RT related operations during a kick are performed, analoguely to hydro_kick_extra
• added option to set upper threshold for star time steps

ToDo before WIP is removed:

• wait for !1460 (merged) to be merged and rebase
• clean up and format code (@everybody else, pls don't look at the current state, it's embarrassing)
• check whether documentation is up to date
• add new function prototypes to all other schemes
• check that you don't break anything else
• discuss addition of rt_additions.h
• update all param files of all examples to include new compulsory parameters so that things at least don't crash at startup

Future ToDos: (not part of this MR)

• update examples (in particular Strömgren Spheres)
• add example to test heating/ionization of gas and energy conservation in a case without radiation sources
• add futher Iliev+09 tests
• add option for user to provide cross sections in parameter file, and script/program to compute them outside swift
• cleaner, leaner way of dealing with ionizing species
• move from explicit cgs in thermochemistry to internal units, test for
• enforce 10% rule recursively during thermochemistry (currently no such rule)
• add theory
• take care of particle drifts