Add dataset Italy Air Quality

[WenjieDu]

What does this PR do?

1. add dataset Italy air quality https://archive.ics.uci.edu/dataset/360/air+quality;
2. return physionet-2012 as three sets separately;

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 9236260675

Details

• 13 of 20 (65.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 81.767%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tsdb/data_processing.py	1	2	50.0%
tsdb/loading_funcs/italy_air_quality.py	4	10	40.0%

Totals
Change from base Build 8313017438:	-1.0%
Covered Lines:	435
Relevant Lines:	532

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💛 - Coveralls