Add CLI command to plot elliptic solver convergence

[nilsvu]

Proposed changes

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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Further comments

[knelli2]

Why not just always have a marker?

[nilsvu]

It's too busy if you have so many markers that you can't distinguish them anymore. And inversely, you should have markers if the line has visible kinks. So 20 seems like a reasonable but fairly arbitrary value in between.