Bottleneck matching warning

[catanzaromj]

This PR adds a UserWarning on matching=True calls due to the previous change swapping out hopcroftkarp dep for scipy. This warning will be removed in a 0.5.0 release.

The warning itself is because when several optimal matchings exist, the bottleneck matching from hopcraftkarp vs scipy may differ. The bottleneck distances themselves are identical, so the warning is only emitted when calling matching=True. This warning was only noted in RELEASE.txt before, so this PR makes it explicit to callers in code.

I also added two tests to make sure the warning is only emitted in the correct case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.87%. Comparing base (096a25f) to head (e19f15a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   78.85%   78.87%   +0.02%     
==========================================
  Files          20       20              
  Lines        1480     1482       +2     
  Branches      270      271       +1     
==========================================
+ Hits         1167     1169       +2     
  Misses        235      235              
  Partials       78       78              

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