added references for the minerbo equations

Source/FlavoredNeutrinoContainerInit.cpp

@@ -38,6 +38,11 @@ Gpu::ManagedVector<GpuArray<Real,3> > uniform_sphere_xyz(int nphi_at_equator){
 
 // residual for the root finder
 // Z needs to be bigger if residual is positive
+// Minerbo (1978) (unfortunately under Elsevier paywall)
+// Can also see Richers (2020) https://ui.adsabs.harvard.edu/abs/2020PhRvD.102h3017R
+//     Eq.41, but in the non-degenerate limit
+//     k->0, eta->0, N->Z/(4pi sinh(Z)) (just to make it integrate to 1)
+//     minerbo_residual is the "f" equation between eq.42 and 43
 Real minerbo_residual(const Real fluxfac, const Real Z){
 	return fluxfac - 1.0/std::tanh(Z) + 1.0 / Z;
 }