ENH: move moran_facet plotting from splot

[martinfleis]

This is nearly 1:1, I just replaced the scatterplot functions for those we already have here and properly specified which spines should be shown and which not. The resulting plot is the exact version of the one splot does.

Tests do not repeat what is tested in scatter itself, so focusing only on the subplot grid and its specification.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (f5c0a56) to head (634e5c5).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #359     +/-   ##
=======================================
+ Coverage   81.5%   81.8%   +0.3%     
=======================================
  Files         24      24             
  Lines       3406    3432     +26     
=======================================
+ Hits        2777    2807     +30     
+ Misses       629     625      -4     

Files with missing lines	Coverage Δ
esda/__init__.py	100.0% <ø> (ø)
esda/moran.py	84.0% <100.0%> (+1.5%)	⬆️