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! 
Giving the full precision is risky, since rounding errors are not the same on all machines. Hopefully, this will be addressed by #2087, but in the meantime, you will have to write something like
```
<Quantity 8.61733238496...e-05 eV>
```