Included velocity power spectra.

Also added automated computation of cross-correlation power spectra P(k),
similarly to what is already done for the C_l's.
Implemented full kSZ formula from eq. 6 of Ma & Fry 2002.

Also corrected a bug in the kSZ kernel functions, which now
fully match those in eq. 7 of Ma & Fry 2002. Note that if using
linear power spectra, there is only a ~1% difference between eq. 6
and eq. 7.