correct the std calculation function and the PyODDataset

[ForestsKing]

1. correct the std calculation function before preprocessing
2. correct the PyODDataset

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[coveralls]

Pull Request Test Coverage Report for Build 2087

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.0007%) to 95.833%

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Totals
Change from base Build 2066:	-0.0007%
Covered Lines:	5428
Relevant Lines:	5664

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💛 - Coveralls