Add the attribute best_epoch to record the best epoch num

[WenjieDu]

What does this PR do?

1. fixing Remind the best model from which epoch after model training #235;

Before submitting

• 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 8602662623

Details

• 15 of 19 (78.95%) changed or added relevant lines in 10 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 80.813%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/clustering/base.py	0	2	0.0%
pypots/forecasting/base.py	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/env.py	2	61.22%

Totals
Change from base Build 8601694436:	-0.001%
Covered Lines:	6798
Relevant Lines:	8412

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💛 - Coveralls