PEP8 check in tests and examples

[ggraffieti]

• Now the pep8 check takes into consideration all the source files inside avalanche/, alongside tests (folder tests/) and examples (folder examples/).
• Fix the existing pep8 issues in those files.
• Deprecated tests removed.
• Delete now useless test utils method (they were used to download the MNIST dataset from a mirror, now integrated into torchvision).

This PR solves issue #499

[coveralls]

Pull Request Test Coverage Report for Build 724053992

• 18 of 19 (94.74%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 76.221%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/test_metrics.py	1	2	50.0%

Totals
Change from base Build 721954277:	-0.1%
Covered Lines:	6475
Relevant Lines:	8495

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💛 - Coveralls