It's disabled by default so that we do not accidentally change the
volume prior in the middle of an observing run.
The WMAP9 cosmological parameters are hard-coded. The prior is
implemented as a cubic spline approximation to the correction
factor relative to a DL^2 prior. This is simple, fast, accurate, and
allows us to use BAYESTAR's already highly tuned distance integrator.
Original: c437d9c1072cbb432e5801bd63285bb1cc49b480