Unit testing exercise - um2nc callback_function()

[truth-quark]

As an initial unit testing exercise, let's start with a standalone function with only inputs/outputs. The um2nc callback_function() is a good one to try: https://github.com/ACCESS-NRI/amami/blob/main/amami/parsers/um2nc_parser.py#L33-L74

Let's split this into parts, so reviews/feedback can be staged between task blocks.

Bonus questions for reviews:

• What was easy?
• What was hard?
• What doesn't work?
• Any general comments or observations?

Part 1 Tasks - Initial unit testing (retrofitting style):

• Configure environment so pytest can be run from the command line & IDE
• Write a unit test to exercise a "good" path through the function & assert the output is correct
• Write a test to go through a failure path
• All tests pass

Can check & review here.

Part 2 Tasks - More unit testing for coverage:

• Write tests such that all good code paths are run through.
• Add tests to cover all failing paths.
• Aim for 100% code coverage.
• All tests pass

Check & review here too.

Part 3 Tasks - Unit testing supporting refactoring:

• Experiment with simplifying & restructuring the code. Can much the code be simplified?

Final wrap up

• Compare old & new side by side (count SLOC, compare readability/clarity/simplicity)