Add simple plotting tool

[wouterjdb]

This PR adds a simple plotting tool which allows you to plot the results of FlowNet runs.

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Contributor checklist

• [x] :tada: This PR closes #148
• [x] :scroll: I have broken down my PR into the following tasks:
  • [x] Add initial version of the tool
  • [x] Make more updates
  • [x] Add observations
  • [x] Add support for multiple vertical lines
• [ ] :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.

[wouterjdb]

Example from @edubarrosTNO