Changed additional flow node input to list to allow various from layer to layer

[wouterjdb]

Changed additional flow node input to list to allow various from layer to layer

This is a breaking change. YOu now have to supply the additional flow nodes as a list (also for a single numer!), like so:

additional_flow_nodes: [1]

or, for two layers,

additional_flow_nodes: [10,50]

Or, alternatively:

additional_flow_nodes:
  - 10
  - 50

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Contributor checklist

• 🎉 This PR closes Problems with layering option for Norne after removing unused well connections #328.
• 📜 I have broken down my PR into the following tasks:
  • Expose additional nodes per layer to user
• 🤖 I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• 📖 I have considered adding a new entry in CHANGELOG.md.
• 📚 I have considered updating the documentation.

A new issue has been created as well #343.