Satistical tests reorganization

[AgnesBaud]

Description

• statistical tests in both statistical_test.py and differential_analysis.py
• SeriesBinning: bin into equal-size bins (in addition to equal-width)

Changelogs

• reorganization Chi2 and other statistical tests
• SeriesBinning: to put series into bins

Issue(s)

Fixes #91

Definition of Done

• [ ] New code is tested with unit tests
• [ ] Documentation has been updated
• [ ] Pull Request has been revised by a maintainer of the project

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (5946124) 65.15% compared to head (cdffb0c) 66.25%. Report is 18 commits behind head on master.

Files	Patch %	Lines
moonstone/analysis/statistical_test.py	95.62%	6 Missing :warning:
moonstone/utils/pandas/series.py	95.55%	2 Missing :warning:
moonstone/plot/graphs/bargraph.py	50.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #92 +/- ## ========================================== + Coverage 65.15% 66.25% +1.10% ========================================== Files 61 61 Lines 2930 3020 +90 ========================================== + Hits 1909 2001 +92 + Misses 1021 1019 -2 ``` | [Flag](https://app.codecov.io/gh/motleystate/moonstone/pull/92/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=motleystate) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/motleystate/moonstone/pull/92/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=motleystate) | `66.25% <95.69%> (+1.10%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=motleystate#carryforward-flags-in-the-pull-request-comment) to find out more.

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