added neural network ensembles

[coveralls]

Pull Request Test Coverage Report for Build 4568592863

• 65 of 71 (91.55%) changed or added relevant lines in 3 files are covered.
• 8 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.3%) to 96.352%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nautilus/sampler.py	3	4	75.0%
nautilus/bounds/neural.py	23	25	92.0%
nautilus/neural.py	39	42	92.86%
<!--	Total:	65	71	91.55%	-->

Files with Coverage Reduction	New Missed Lines	%
nautilus/sampler.py	8	93.36%
<!--	Total:	8		-->

Totals
Change from base Build 4515394203:	-1.3%
Covered Lines:	1030
Relevant Lines:	1069

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💛 - Coveralls