Allow maxabs to handle inputs with NaNs

[leouieda]

verde.maxabs now uses the nan* version of numpy functions by default
to handle arrays with NaNs (like masked grids). This can be turned off
with an argument in case the not nan behavior is desired.

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 verde/__init__.py.
• Write detailed docstrings for all functions/classes/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.