ENH: moved probplot over to properties and added some kwargs to plotting fxns
Also reworked the tests.

BUG: fixed a 'call' to a property

BUG: fixed missing paren in sample_percentile equation

WIP: trying out this cache_readonly decorator

TST: beefier, class-based tests

TST: decorating methods, not test classes + fixed a function call

TST/BUG: fixed references to class attributes

TST/BUG: fixed references to class attributes (part2)

TST/BUG: using the right type of cache

TST/BUG: fixed references to class attributes (part3)
MNT: doc string formats, error raising, and kwarg order per comments in PR
MNT: test class names (capitalized) and trailing spaces
MNT: more trailing space
DOC/TST: fixed from docstring styles/typos and moved therandom seed to the setup methods of the tests
Merge pull request #1179 from phobson/ProbPlot-enhancements-and-tests

REF/TST: `ProbPlot` now uses `resettable_cache` and added some kwargs to plotting fxns