Allow saving additional information returned by the likelihood function

[segasai]

Starting implementation of #368 The function can now return an additional blob The tests should pass, but the additional info is not saved yet. early feedback welcome

[coveralls]

Pull Request Test Coverage Report for Build 3108342513

• 82 of 85 (96.47%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 90.576%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/dynesty/dynamicsampler.py	33	34	97.06%
py/dynesty/utils.py	23	25	92.0%
<!--	Total:	82	85	96.47%	-->

Totals
Change from base Build 3088827015:	0.06%
Covered Lines:	3950
Relevant Lines:	4361

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💛 - Coveralls

[segasai]

This is now a workign version that allows the likelihood to return an arbitrary 1d numpy array (optionally) if blob=True option is specified in the sampler. Those are stored in results.blob attribute the example is in the test/test_blob.py the docs are absent It is unclear whether the name for the 'blob' is the best one, the tests need to be expanded to include pools and resuming.

[segasai]

I think this is ready for review. It'd be great to have a second look on this. After excluding docs/tests the patch is surprisingly small and not too difficult. The performance effects are probably non-zero given the overhead of objects etc (at least the test suite on my machine seemed to slow down by ~ 2.5%, but that's probably small enough to not worry about it) Also I am thinking of making the next version 2.0 given this change and the checkpointing..