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

---

Contributor checklist

• [x] :tada: This PR closes #328.
• [x] :scroll: I have broken down my PR into the following tasks:
  • [x] Expose additional nodes per layer to user
• [x] :robot: I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• [x] :book: I have considered adding a new entry in CHANGELOG.md.
• [ ] :books: I have considered updating the documentation.

A new issue has been created as well #343.