Fix 3D bounds

[Chilipp]

This PR fixes the visualization of polygons and data grids with more than 2D bounds.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   83.13%   83.20%   +0.07%     
==========================================
  Files           9        9              
  Lines        4786     4800      +14     
==========================================
+ Hits         3979     3994      +15     
+ Misses        807      806       -1     

Impacted Files	Coverage Δ
psy_simple/plotters.py	83.26% <100.00%> (+0.06%)	⬆️
psy_simple/plugin.py	78.57% <100.00%> (+0.50%)	⬆️

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