update random number generation to numpy.random.Generator

[coveralls]

Pull Request Test Coverage Report for Build 4674259311

• 66 of 72 (91.67%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 96.104%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nautilus/bounds/union.py	10	11	90.91%
nautilus/sampler.py	18	19	94.74%
nautilus/bounds/basic.py	26	28	92.86%
nautilus/bounds/neural.py	12	14	85.71%
<!--	Total:	66	72	91.67%	-->

Totals
Change from base Build 4618941984:	-0.4%
Covered Lines:	1036
Relevant Lines:	1078

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💛 - Coveralls