(Closes #1280) fix ref guide warnings

[arporter]

No description provided.

[arporter]

The CI failed at the package installation stage:

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
jupyterlab-server 2.24.0 requires importlib-metadata>=4.8.3; python_version < "3.10", but you have importlib-metadata 4.2.0 which is incompatible.

I don't think this is the fault of this PR. Possibly something has changed in the GHA environment?

[arporter]

In fact, the CI doesn't test the Jupyter notebooks anymore as they are out of date. We could probably therefore relax our dependencies?

EDIT: in fact we don't have jupyter as a dependence and I don't know what requires importlib-metadata to be pinned?

I think this is related to the fact that Python 3.8 has reached EoL. I'll create a new issue - #2777.

[arporter]

Another update. The CI installs jupyter itself (i.e. it's not in setup.py) and then does run the notebooks in examples/gocean/eg1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.88%. Comparing base (9f4890a) to head (551a9fb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2776   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files         356      356           
  Lines       49446    49480   +34     
=======================================
+ Hits        49388    49422   +34     
  Misses         58       58           

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