add a dynesty pool

[segasai]

This PR addresses #393

[coveralls]

Pull Request Test Coverage Report for Build 3077894332

• 41 of 45 (91.11%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 90.648%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/dynesty/pool.py	40	44	90.91%
<!--	Total:	41	45	91.11%	-->

Totals
Change from base Build 3062417057:	0.05%
Covered Lines:	3887
Relevant Lines:	4288

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💛 - Coveralls

[segasai]

I think this is ready for the merge. I'll aim to merge in a day or two.