Add mean and median as imputation methods, and update docs

[WenjieDu]

What does this PR do?

1. add Mean and Median;
2. update the docs;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 8340258469

Details

• 83 of 93 (89.25%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 81.762%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/mean/model.py	37	42	88.1%
pypots/imputation/median/model.py	37	42	88.1%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/env.py	1	65.31%

Totals
Change from base Build 8323647996:	0.1%
Covered Lines:	5205
Relevant Lines:	6366

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💛 - Coveralls