introduce map background for MapPlot

[akrherz]

refs #304

[codecov]

Codecov Report

Base: 97.35% // Head: 97.37% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (92bafb0) compared to base (be69f6d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #697      +/-   ##
==========================================
+ Coverage   97.35%   97.37%   +0.01%     
==========================================
  Files          80       80              
  Lines        9316     9359      +43     
==========================================
+ Hits         9070     9113      +43     
  Misses        246      246              

Impacted Files	Coverage Δ
src/pyiem/plot/_mpl.py	100.00% <100.00%> (ø)
src/pyiem/plot/geoplot.py	95.84% <100.00%> (+0.02%)	⬆️
src/pyiem/plot/util.py	100.00% <100.00%> (ø)

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