add a simpler version of neutrino cooling

[zingale]

Kippenhahn & Weigert have very simple expressions for the neutrino terms (Eqs. 18.81 to 18.86). It would be good to add a simpler neutrino routine with those. What we are using might be more than is needed.

[zingale]

It looks like Haft, Raffelt, and Weiss 1994 give a fit for plasma neutrinos that they claim is accurate:
https://articles.adsabs.harvard.edu/pdf/1994ApJ...425..222H

it is equations 23 and following