Feature/pyflow 22 pyflow extern Limit ?

[morianemo]

Description

https://jira.ecmwf.int/browse/PYFLOW-22

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📚 Documentation preview 📚: https://pyflow-workflow-generator--88.org.readthedocs.build/en/88/

[corentincarton]

@morianemo, I cleaned up the extern and also the way the repeat were created. A node has now a repeat attribute, which is also inherited from the parent.
I also changed the way the follow attribute works. It can now accept a node, meaning we can use the follow in a more granular way.