support Colormap object and kwargs in plot_legendgram

[martinfleis]

Allow Colormap class alongside string as a cmap and pass kwargs to ax.hist for further customisation. Also includes @QuLogic's comments from #234.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.0%. Comparing base (57990b2) to head (2c25144).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #250   +/-   ##
=====================================
  Coverage   90.0%   90.0%           
=====================================
  Files         10      10           
  Lines       1169    1170    +1     
=====================================
+ Hits        1052    1053    +1     
  Misses       117     117           

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

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