Jamboree dimensionality reduction metrics

[lazappi]

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• The task addresses a valid open problem in single-cell analysis
• The latest version of master is merged and tested
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Description

This PR contains the following new dimensionality reduction metrics from the jamboree:

Metric	Author	Issue	Original PR
NN co-ranking	@michalk8	#300	lazappi/pull/8
Local density	@michalk8	#291	lazappi/pull/7