remove_stubs can drop countries

[davide-f]

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-earth environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

Running remove_stubs may drop entire countries if few buses of the country are identified by remove_stubs.
This behavior can be solved by setting the following option to false by default.

pypsa-earth/config.default.yaml

Line 65 in 6bf4b1c

remove_stubs_across_borders: true

This is not a priority task and I keep it open, it is a nice first good issue.
To fix we need to just change the option to false in the config.tutorial.yaml and config.default.yaml files.