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! 
It's simpler than that: just multiply with `k_B` to make an energy (strictly, the division by `e` is not needed, but since the temperature is usually quoted in `eV` and not in `J`, I think this makes clearer what the equivalency is about). Anyway, given that you wondered, it would be good to be clear.