Adding the ETT dataset

[WenjieDu]

What does this PR do?

Adding the dataset https://github.com/zhouhaoyi/ETDataset

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.

[github-actions]

Pull Request Test Coverage Report for Build 7271568346

• 7 of 17 (41.18%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.2%) to 63.125%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tsdb/data_processing.py	1	2	50.0%
tsdb/loading_funcs/electricity_transformer_temperature.py	4	13	30.77%

Totals
Change from base Build 6926376300:	-1.2%
Covered Lines:	303
Relevant Lines:	480

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💛 - Coveralls