Add informer as an imputation model

[WenjieDu]

What does this PR do?

1. add Informer as an imputation method, as suggested in New Model #295;

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 8519954763

Details

• 208 of 254 (81.89%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 80.807%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/informer/model.py	66	70	94.29%
pypots/imputation/informer/modules/submodules.py	101	143	70.63%

Totals
Change from base Build 8519753978:	0.2%
Covered Lines:	6787
Relevant Lines:	8399

---

💛 - Coveralls