Save training logs into tensorboard files

[WenjieDu]

I add implementations for each task and model to help save training logs into tensorboard files. Test cases to validate saving files are also added.

[coveralls]

Pull Request Test Coverage Report for Build 4666681241

• 247 of 262 (94.27%) changed or added relevant lines in 17 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+1.2%) to 86.033%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/classification/brits.py	14	15	93.33%
pypots/clustering/base.py	0	1	0.0%
pypots/classification/raindrop.py	2	4	50.0%
pypots/clustering/vader.py	9	12	75.0%
pypots/forecasting/base.py	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/clustering/base.py	1	35.53%
pypots/clustering/vader.py	1	91.84%
pypots/data/base.py	1	79.59%
pypots/forecasting/base.py	1	30.49%

Totals
Change from base Build 4656151815:	1.2%
Covered Lines:	2852
Relevant Lines:	3315

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💛 - Coveralls