Custom clustering

[whalebot-helmsman]

In this PR:

• fix for default_clustering_score to reduce score for clustering with threshold=0
• ability to pass custom distance and position functions to clustering procedure
• new _get_tree_position and _get_tree_distance functions provide much better grouping results

I didn't set these new functions as default functions. Using them as default will improve grouping quality.

[codecov[bot]]

Codecov Report

Merging #66 into master will decrease coverage by 0.59%. The diff coverage is 68.18%.

@@            Coverage Diff            @@
##           master      #66     +/-   ##
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- Coverage   81.02%   80.43%   -0.6%     
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  Files          40       40             
  Lines        2092     2131     +39     
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+ Hits         1695     1714     +19     
- Misses        397      417     +20