Deprecate the scatter method of BaseGridder

[leouieda]

The method is a bit useless since we rarely want to predict on random
points. Copy the code to the CheckerBoard function since that's where
it is most useful. Issue a FutureWarning from the base scatter
method.

TODO:

• Add a test for the new scatter method in CheckerBoard
• Check the documentation for use of scatter

Fixes #291

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.