The attributes argument for the sklearn.utils.validation.check_is_fitted
function is going to be deprecated. Remove the attributes passed to the
check_is_fitted function used on the predict method of every predictor
from Verde. Modify the example on verde.base.BaseGridder to show how the
check_is_fitted function must be used when subclassing the BaseGridder.

Fixes #216

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.