Add example scripts image test coverage

[bjlittle]

🚀 Pull Request

Description

Todo:

• Check whether using xvfb and software rasterizer MESA on GHA runner affects test image tolerances
• Relocate image_cache images to geovista-data repo
• Support pooch download of image_cache assets
• Extend CLI

---

[codecov]

Codecov Report

Merging #494 (d0e44de) into main (44edaeb) will increase coverage by 4.21%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##             main     #494      +/-   ##
==========================================
+ Coverage   90.74%   94.95%   +4.21%     
==========================================
  Files          70      105      +35     
  Lines        3686     4484     +798     
==========================================
+ Hits         3345     4258     +913     
+ Misses        341      226     -115     

Files	Coverage Δ
src/geovista/__init__.py	90.90% <100.00%> (+0.90%)	⬆️
src/geovista/cache.py	89.79% <100.00%> (+4.08%)	⬆️
src/geovista/examples/clouds.py	96.15% <ø> (ø)
src/geovista/examples/clouds_robin.py	96.42% <ø> (ø)
src/geovista/examples/earthquakes.py	80.00% <ø> (-0.77%)	⬇️
src/geovista/examples/earthquakes_wink1.py	80.76% <ø> (ø)
src/geovista/examples/from_1d__oisst_eqc.py	95.45% <ø> (ø)
...geovista/examples/from_1d__synthetic_face_m1_n1.py	95.65% <100.00%> (ø)
...ta/examples/from_1d__synthetic_face_m1_n1_robin.py	96.00% <100.00%> (ø)
...geovista/examples/from_1d__synthetic_node_m1_n1.py	95.65% <100.00%> (ø)
... and 23 more

... and 14 files with indirect coverage changes