Vertical CRS download fails on "Cheatsheet" documentation page

[rhugonnet]

Only fails on ReadTheDocs (works fine locally), and only on that page. It also works on the "Vertical referencing" page on the ReadTheDocs, even though it uses the same transformation... 🤯

I tested a few thing (copy/pasting transformation of the "Vertical referencing" page, changing vertical CRS), without success...

[rhugonnet]

Finally found more on this mysterious issue: The downloading only fails for the first notebook executed by Readthedocs that calls DEM.to_vcrs() (Reminder: it works fine locally).
By default, this is alphabetical, so cheatsheet.md fails. If I rename it into something starting with "z", it's instead the dem_class.md that fails. And then vertical_ref.md.
I'm trying to circumvent by a trick for now, adding an occurence of to_vcrs() in a fully hidden cell of biascorr.md (the first notebook to be executed).
If that works, I'll merge the temporary fix like this.

[rhugonnet]

The temporary fix in #666 works but is not ideal (adds useless code to biascorr.md, might break if we add a new page that executes before alphabetically), so re-opening this to keep track of it.