Radiation 2D - solves #289

adjust radiation kernel to work in 2D
solves #289 

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~~**Do not merge yet!**~~
Update:
**tests were positiv**
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~~Currently, simulations are waiting on hypnos for verifying the physics/radiation results.  ~~
~~If they produce correct results, this branch can be merged.~~
**Update**: they did.
Up until then, feel free to comment on the code.
Thanks @PsychocoderHPC for the support on transferring the code to 2D.