Adding TimesNet as an imputation model

[WenjieDu]

What does this PR do?

Fixing #250;

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).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's 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 7062000341

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• 240 of 265 (90.57%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 83.793%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/timesnet/modules/layer.py	63	66	95.45%
pypots/imputation/timesnet/model.py	67	74	90.54%
pypots/imputation/timesnet/modules/embedding.py	58	73	79.45%

Totals
Change from base Build 7061808206:	0.4%
Covered Lines:	4710
Relevant Lines:	5621

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💛 - Coveralls