support of parallelization of sampling

[coveralls]

Pull Request Test Coverage Report for Build 4689422173

• 93 of 132 (70.45%) changed or added relevant lines in 5 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-3.2%) to 92.883%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nautilus/bounds/union.py	25	30	83.33%
nautilus/bounds/basic.py	7	17	41.18%
nautilus/bounds/neural.py	29	53	54.72%
<!--	Total:	93	132	70.45%	-->

Files with Coverage Reduction	New Missed Lines	%
nautilus/bounds/basic.py	1	93.91%
nautilus/neural.py	1	93.22%
<!--	Total:	2		-->

Totals
Change from base Build 4674316141:	-3.2%
Covered Lines:	1044
Relevant Lines:	1124

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💛 - Coveralls