Release v0.4, apply SAITS embedding strategy to the newly added models, and update README

[WenjieDu]

What does this PR do?

1. apply SAITS embedding strategy to the newly added models as we discussed in New Model #295;
2. update README;
3. release v0.4;

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 8615956810

Details

• 80 of 81 (98.77%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 81.03%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/dlinear/modules/core.py	33	34	97.06%

Totals
Change from base Build 8607870864:	0.2%
Covered Lines:	6847
Relevant Lines:	8450

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💛 - Coveralls