RT: rudimentary thermochemistry tests

This MR adds two very basic tests for RT:

• A uniform initially hot box without radiative sources nor any radiation which is supposed to cool down. The purpose of this test is to check whether the cooling without any radiation works as expected.
• A uniform initially cold/warm box with a fixed amount of initial radiation which is supposed to heat up the gas. The purpose here is to check whether the interaction between radiation and gas is done properly, i.e. that the gas energy increases while the radiation energy decreases.

For the cooling example, I've prepared the expected solution using a grackle standalone. It would be awesome if somebody could add it to the cosma file server so we can fetch that reference using the provided script. Here is the file: RTCoolingTestReference.txt. I set the link in the script following other conventions as wget http://virgodb.cosma.dur.ac.uk/swift-webstorage/ReferenceSolutions/RTCoolingTestReference.txt.

For the heating test, I don't think that an expected solution would be of much benefit. On one hand, if I created it using grackle, I'd have to compute a lot of things regarding the interaction rates manually, repeating potential mistakes again and getting identical results. On the other hand, the entire goal is to just check energy conservation. Which, ironically, currently isn't the case in my scheme because I haven't implemented the re-injection of recombination radiation after the thermochemistry step (yet).

Some minor additional changes in this MR:

• Added licenses to python RT scripts
• added an additional task count in engine_estimate_nr_tasks for the new collect task which solves all "Task list overflow(N). Need to increase Scheduler:tasks_per_cell" issues on small examples I encountered
• added simple estimate of "cooling time" for GEAR RT
• some minor cleanups in GEAR RT