Transfer function
* 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
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