decide on a policy for using fixtures

Fixtures provide a convenient way to set up tests, but there are alternatives as well. Most of what they achieve could also be achieved by calling regular functions, or by using pytest cases. In fact, all three of these approaches are used in our testing code.
We should investigate which approach brings what benefits and what disadvantages, and come up wit a policy for choosing the approach.
We can use other python projects for reference/inspiration (such as xarray), read testing framework documentation, and discuss the findings.

Review the existing testing code to compare fixtures, regular function calls, and pytest cases, then consult testing framework documentation and the xarray project for reference. Done means producing and discussing a clear policy that explains the benefits and disadvantages of each approach and when to choose one.