Use doctest to test documentation code examples

[thorbjoernl]

Is your feature request related to a problem? Please describe.
The Pyaerocom documentation contains a number of code examples as a part of its documentation, however we currently have no way of verifying that they work, or that they will work in the future.

I suggest we add doctest to include these snippets to the existing testing pipeline (maybe we could even rewrite the tutorials so they are tested?)

This would ensure that:

• Code examples are self-contained.
• That code examples produce the expected output also in future versions of Pyaerocom.
• It may improve test coverage.

Describe the solution you would like to see

• Would you be able to work on this solution yourself? Sure
• How can the Pyaerocom development team assist you?

Additional context

• Setting up the testing is pretty trivial but I suspect it will be quite some work to get existing code examples to work correctly.
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