Change central_longitude to 0 to workaround upstream Cartopy issue #2488 and fix CI - #535

Summary

I noticed the CI has been failing since last release:

• The error is ValueError: 'vertices' must be 2D with shape (N, 2), but your input has shape (0,) in examples/notebooks/plotting-maps.ipynb.
• This is an upstream Cartopy issue ValueError: 'vertices' must be 2D with shape (N, 2), but your input has shape (0,) SciTools/cartopy#2488 introduced in v0.24 and fixed in return valid (empty) path if path contains no polygons SciTools/cartopy#2619 but hasn't been released yet.
• Downgrading Cartopy to v0.23 is not an option since it is not available in Python 3.14 on conda-forge.
• The easiest workaround is to change the central_longitude from 240. to 0. for CI to pass. The other cells with this value may be updated for consistency.
• I just included the code change for now. The images in the notebook need to be regenerated.

Reminders

• Base all changes on the develop branch: the master branch is used only when releasing new versions.
• Run make check to ensure that the python code follows standard formatting conventions.
• If adding new features, update the docstring to provide all information that is required to use the feature.