Improved worksharing across threads for find_overlaps_groups.

[LTLA]

The idea is to ensure that all threads are occupied, even when the groups are of different sizes. This is done with some more fine-grained parallelization during both index building and overlap identification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.04%. Comparing base (b0d06bb) to head (e580aab).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   67.04%   67.04%           
=======================================
  Files           4        4           
  Lines         877      877           
  Branches      166      166           
=======================================
  Hits          588      588           
  Misses        213      213           
  Partials       76       76           

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[jkanche]

seems like it slows down the runtimes. not sure what might be causing this, I will investigate this later

[LTLA]

Well the previous code didn't parallelize at all if there was only a single group. So this should actually make use of multiple threads and be faster in that scenario.

[jkanche]

gotcha, arigatou guzaimasu