WIP/BUG: fixed ProbPlot class and tests to work with general distributions

Unsurprisingly, most of what I did to get this point was to simplify things.

Here's a notebook showing how things are working so far. It shows off creating `ProbPlot` objects with three different distributions:
 1. Lognormal
 1. beta
 1. t

It also creates the object three different ways:
 1. Simple: provide data, unfrozen distribution, minimal distribution parameters (`distargs`)
 1. More specific: provide data, frozen distribution, minimal distribution parameters (`distargs`)
 1. "Automatic": provide data, unfrozen distribution, tell the class to fit the distribution

Not all the plots are perfect. But we're getting close.