Fixing the infinite loop in LOCF

[WenjieDu]

…first_step_imputation;

What does this PR do?

Fixing #281

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 7301230056

• 24 of 27 (88.89%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 81.77%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/base.py	1	2	50.0%
pypots/imputation/locf/model.py	23	25	92.0%

Totals
Change from base Build 7299434774:	-0.04%
Covered Lines:	5091
Relevant Lines:	6226

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💛 - Coveralls