[MRG+1] - Refactor seasonality module

Description

Primary purpose here is to refactor the CHTest() class to add a new method _calc_ch_crit_val(). This brings it in-line with the OCSBTest() class, and will enable the user to extract the specific critical value used for the test.

The functionality does not change, all other processes remain the same.

While adding this change, I ran the black formatting on the seasonality module, which caused quite a few changes.

Type of change

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation change
[x] Refactoring

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

[ ] Test A
[ ] Test B
[x] Unit tests all passing

Checklist:

[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation (no need, as internal methods are not generated on the docs page)
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works (nothing added, so no new tests are needed)
[x] New and existing unit tests pass locally with my changes

alkaline-ml / pmdarima