kelvin to eV

Hello all. 
This pull request adds a new equivalency to convert between kelvin and eV. In my view this conversion comes to play in many occasions and is pretty useful. 
If something needs to be added further, please let me know! 
Thank you! 
Also, it should be `t_k` in both lines; personally, I'd also pick a more meaningful temperature, say, `1e7 K` or so, and convert that to `keV`.