Switch default 3pt to use LogSAS and ordered=True

[rmjarvis]

This is more prep for the multipole algorithm. That algorithm will need to use LogSAS binning, not LogRUV, and since it is so much faster than the current 3 cell recursive algorithm, it makes sense to switch the default to be LogSAS. LogRUV is still available, but I don't expect most people to want to use it.

Also, I realized that the new ordered=True option to process is likely to be the version that most people will want for most applications. I think the only common use case that would prefer ordered=False is when doing data-random cross correlations, so I explicitly call out that case as an example in the docs.

Finally, there are some other minor cleanup changes that I ran into when working on the multipole algorithm, so I'm putting them here to keep that PR as clean as possible.