Statannotations

- Major refactoring, change to an Annotator `class` to prepare and add
annotations in separate function (now method) calls.
- Support for `violinplot`
- Fixes in rendering of non-significant tests results after multiple
comparisons correction
- Fix the printout of the star annotation legend
- Fix the failure when providing a dataframe with categories as different
columns.
- Fix a bug where an incorrect xunits calculation resulted in wrong
association of points within a box, leading to erroneous max y position for
that box.
- Reduced code complexity, more SOLID.
- Many unit and integration tests

- Fix `simple` format outputs
- Fix `ImportError` when applying a multiple comparison correction without
statsmodels.
- Multiple comparison correction is `None` by default, as `statsmodels` is an
additional dependency.

- Added support of functions returning more than the two expected values when
used in `StatTest`
- Fix version numbers in CHANGELOG

(Explicitly Requires Python 3.6)

- Refactoring with implementation of `StatTest`
([7](https://github.com/trevismd/statannotations/pull/5)), removing the
`stat_func` parameter, and `test_long_name`.
- Annotations y-positions based on plot coordinates instead of data
coordinates
(PR [5](https://github.com/trevismd/statannotations/pull/5) by
[JosephLalli](https://github.com/JosephLalli),
fixes https://github.com/webermarcolivier/statannot/issues/21).
- Add this CHANGELOG

- Fix bug on group/box named 0, fixes
https://github.com/trevismd/statannotations/issues/10, originally in
https://github.com/webermarcolivier/statannot/issues/78. Independently
fixed in https://github.com/webermarcolivier/statannot/pull/73 before this
issue.