PROBABILITY OF should yield log densities, not densities

[riastradh-probcomp]

Why not just do it now? From discussion during code review of an unrelated change to PROBABILITY OF:

It is a major incompatible change to the semantics of the language.

I wouldn't object to making that change provided we carefully made sure all callers were ready to accept it in bayeslite (including Geweke testing), bdbcontrib, bdb-experiments, &c. We should probably also run it by Vikash.

Relevant: https://github.com/probcomp/bayeslite/issues/114