Improved analysis API to make it easier to use programmatically

The current analysis API produces a catalog, but this is not always convenient to use in a programmatic way. With minimal changes to the current API, I've make ``PPStatistic`` and ``PPVStatistic`` public, and have added documentation on using them - in particular, see the example for why this kind of API is useful. Here's a preview of the documentation page:

https://dendrograms-astrofrog-v2.readthedocs.org/en/latest/catalog.html

Note that there was a bug in the definition of the major/minor axes - the second moment needs to be multiplied by two to give the actual major/minor axes.

I'd like to make the statistics properties rather than methods of the ``Statistic`` classes. Is there any reason not to do this?