Use keywords for default arguments in tests and examples

[santisoler]

Explicitly pass default arguments with their corresponding keywords on tests
and examples. Particularly for grid_coordinates and BaseGridder.grid calls.
It's better if we instruct people to use keywords of default arguments instead
of passing them by order, this way can add or change the order of default
arguments without breaking backward compatibility.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

👍🏽

We should think about making our functions/methods take keyword-only arguments. That would have avoided this type of thing.