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! 
I would just calculate `y = (constants.k_B * x).to(u.keV)` [this not being the place to check that constants are defined correctly].