99.6% hull.py test coverage

[kimstik]

Added tests for the bits of hull.py nothing was hitting.
Two of them found real bugs along the way - a lone circle didn't work,
and one input hung forever - so there's a small fix in "find_hull" too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.80%. Comparing base (cb26a27) to head (268a99a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2096      +/-   ##
==========================================
+ Coverage   95.76%   95.80%   +0.04%     
==========================================
  Files          30       30              
  Lines        9584     9589       +5     
  Branches     1437     1439       +2     
==========================================
+ Hits         9178     9187       +9     
+ Misses        250      247       -3     
+ Partials      156      155       -1     

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