Add tests/clean fix_level_coord()

[truth-quark]

Closes #53. Note, this PR is merges to main.

This PR:

• adds unit tests for fix_level_coord()
• moves the function down to call order (i.e. below process())
• Cleans the code slightly

These tests use iris coordinate objects to avoid mocks cube attributes. Based on this experiment, this could be a reasonable approach for other tests requiring iris coordinates.

Any comments welcome!

[blimlim]

Looks good! The only change is the fix_level_coord docstring, while the other two are questions about the clarity for the tests and so aren't completely necessary.

[truth-quark]

I've refactored the tests with fixtures, now the assertions test against a cube.coord("name") function call interface. It has resulted in some 1st class object passing, but it's constrained within a fixture. There's a potential future benefit as the fixture dataclass can be called with init args in tests.

@blimlim How do you find the readability of this change?

[blimlim]

Changes look good and clearer to read!