Uniform leaf node positioning on lineage graph

[jules-vanaret]

This PR modifies the function that positions nodes on the 2D lineage graph so that leaf nodes are uniformly positioned along the horizontal axis, i.e the horizontal gap between two consecutive leaf nodes on the lineage graph is a constant across all pairs of consecutive leaf nodes.

This has the effect of optimizing the space spanned by the lineage graph, so that lineages with many divisions are not squished by "shallow" lineages. The idea is illustrated on the following image:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.20%. Comparing base (f835b62) to head (366d0e0).
⚠️ Report is 8 commits behind head on v3.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             v3.x      #86      +/-   ##
==========================================
+ Coverage   86.00%   86.20%   +0.20%     
==========================================
  Files          20       20              
  Lines        1765     1791      +26     
==========================================
+ Hits         1518     1544      +26     
  Misses        247      247              

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