Sample mesh plotting code has broken

[pp-mo]

🐛 Bug Report

How To Reproduce

Steps to reproduce the behaviour:

1. copy the sample code from the UserGuide examples in "further_topics/ugrid/operations#load" and "further_topics/ugrid/operations#plotting"
2. insert an unstructured netCDF/UGRID test file, such as "iris-sample-data:mesh_C4_synthetic_float.nc"
3. run the code to produce a plot

Expected behaviour

Code should be self-contained, and it should run

Problems

• Primarily, the from geovista.common import to_xyz line is now failing, since this geovista routine now got renamed to "geovista.common.to_cartesian"
• Secondarily, the resulting plot is now near-empty because the coarse data is obscured by the base_layer (e.g. a radius change is needed to avoid one obscuring the other). It's not clear whether this changed, or maybe did not occur with the data used in the original (we don't know what that was)

There are obviously also some related strategic problems which this highlights:

• the example is not run, so no way to spot when it goes stale (and in this case, now hits an exception)
• the example is not self-contained, so you can't simply "run it" anyway -- and neither can a user

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[trexfeathers]

I think #5740 took care of this. @pp-mo please re-open if you disagree.