Add pairwise anisotropy profile calculation

[aphearin]

To support ongoing work with @sssaito, and in analogy to mean radial velocity profiles and radial velocity dispersion profiles, include a new cython engine and user-facing convenience function to calculate the anisotropy profile

\beta^2(r) = 1 - 0.5(sigma_t^2 / sigma_r^2)

and also

\beta^2(r/Rvir) = 1 - 0.5(sigma_t^2 / sigma_r^2)

CC @manodeep - have you implemented this in Corrfunc?

[manodeep]

Nope. No plans either