Approx prior

This series of commits (building on !1 (merged)) fits a rational-function of dL to a uniform-in-source-frame merger rate to produce a simple, analytic dL prior that matches well with the true uniform-in-source-frame merger rate out to z ~ 4. An option is added to the reweighting script to un-do this prior at the same time as adjusting the masses to uniform in source frame.

Maya, once you have checked the approxprior.py module and associated notebook and Daniel W. has merged in the updates to the reweighting script, go ahead and merge.