Update halofit

Update the model of Halofit used to match that of arXiv:1208.2701, which
works better on small scales and incorporates a correction for dark
energy.

Also update the massive neutrino fitting formula parameters so they work
atop of the above modified halofit. These parameters have been generated
using the same procedure as in arXiv:1109.4416 and give similar results,
but assume the newer halofit.

The same halofit parameters are also now used in CAMB.