Consider changing default behavior of pathfinder num_paths=1

[WardBrian]

When pathfinder was first being exposed, it was decided that num_paths=1 would run single-path pathfinder. This was at the time the only way to turn off PSIS resampling.

Now that we have the argument psis_resample, we could do this another way, but that requires changing the default. In todays Stan The Gathering, @avehtari @mitzimorris and @bob-carpenter all voted in favor of making this change