Add the method predict() for all models

[WenjieDu]

What does this PR do?

Fixing #198

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 6414628284

• 214 of 224 (95.54%) changed or added relevant lines in 29 files are covered.
• 8 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.8%) to 84.577%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/classification/base.py	2	3	66.67%
pypots/clustering/base.py	2	3	66.67%
pypots/clustering/crli/model.py	11	12	91.67%
pypots/clustering/vader/model.py	11	12	91.67%
pypots/forecasting/base.py	2	3	66.67%
pypots/imputation/base.py	2	3	66.67%
pypots/base.py	18	22	81.82%

Files with Coverage Reduction	New Missed Lines	%
pypots/classification/base.py	2	78.72%
pypots/clustering/base.py	2	38.37%
pypots/forecasting/base.py	2	35.16%
pypots/imputation/base.py	2	80.0%

Totals
Change from base Build 6333946445:	0.8%
Covered Lines:	3932
Relevant Lines:	4649

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💛 - Coveralls