cmap detection functionality

[jGaboardi]

This PR massages in the remaining functionality that is present in pysal/legendgram#11. This is the detection of a colormap on the input ax, something that @slumnitz suggested and I implemented in there.

See this thread and this gist as a minimal example of the internal _get_cmap() function.

Once this PR is merged pysal/legendgram#11 can be closed out, which will close out pysal/legendgram#6 and pysal/legendgram#8, leaving only the actual archival of legendgram to be done – pysal/legendgram#22

cc @ljwolf

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.1%. Comparing base (cb25d35) to head (4e3d0ec).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #273     +/-   ##
=======================================
+ Coverage   90.0%   90.1%   +0.1%     
=======================================
  Files         10      10             
  Lines       1170    1185     +15     
=======================================
+ Hits        1053    1068     +15     
  Misses       117     117             

Files with missing lines	Coverage Δ
mapclassify/classifiers.py	88.6% <ø> (ø)
mapclassify/legendgram.py	100.0% <100.0%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.