Update for v0.3.1

[kayzliu]

• add edge drop probability to structural outlier injection.
• update benchmark script with more datasets.
• multiple minor fixes by @cshjin @YingtongDou @kayzliu

[coveralls]

Pull Request Test Coverage Report for Build 3002897101

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 89.892%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pygod/generator/outlier_generator.py	4	6	66.67%
<!--	Total:	4	6	66.67%	-->

Totals
Change from base Build 2969894681:	-0.04%
Covered Lines:	3593
Relevant Lines:	3997

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💛 - Coveralls