Transfer function: P=knT2(k)

Key scales:

* Horizon at zeq : 16 (Wmh2)-1 Mpc (observe Wmh)

* Free-stream length : 80 (M/eV)-1 Mpc (Wm h2 = M / 93.5 eV)

* Acoustic horizon : sound speed < c/31/2

* Silk damping

M sets damping scale - reduced power rather than cutoff if DM is mixed

Generally assume adiabatic

Parameters: Wd Wb Wv Wneutrino h w n M