Radiation 2D - solves #289

adjust radiation kernel to work in 2D
solves #289 

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**Do not merge yet!**
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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.
Up until then, feel free to comment on the code.
Thanks @PsychocoderHPC for the support on transferring the code to 2D.
Thanks for the explanation. :+1: