effective spin distributions
Add distributions over effective spins (at least chi_eff). Support two types of distributions
- p(chi_eff|q)
- p(chi_eff|mass1_source, mass2_source)
Specifically, support
- IsotropicPowerLawMagnitudeChiEff (see LIGO-P2100112)
- BetaChiEff
- TruncatedGaussianChiEff
Additionally add support to automatically compute "draw probability" needed to importance sample over chi_eff from distributions over the component spins (see LIGO-T2200294). Do this by using the draw probability over component spins, IsotropicPowerLawSpin1Spin2 (with appropriate parameters), and IsotropicPowerLawMagnitudeChiEff in the appropriate combination.