Deprecate the sample datasets

[santisoler]

Sample data will be removed in Verde 2.0.0 in favour of using the Ensaio library instead. Add a deprecation warning to all fetching and plotting functions and warnings to the documentation and docstrings.

Fixes #276

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.

[santisoler]

After removing these functions, the dataset module will only contain the synthetic CheckBoard class.
Shouldn't we find a new place for it to live, maybe something like verde/synthetic (as Harmonica has)?

[santisoler]

We should also update the gallery examples to use Rockhound fetch functions instead the ones in verde.datasets

[leouieda]

@santisoler I updated this to mention Ensaio instead and will merge it as soon as CI are happy. Thanks for pushing this!

[leouieda]

After removing these functions, the dataset module will only contain the synthetic CheckBoard class.
Shouldn't we find a new place for it to live, maybe something like verde/synthetic (as Harmonica has)?

It probably would be better there. We'd need a new PR adding the verde/synthetic.py module with CheckerBoard and import it back in verde.datasets with a DeprecationWarning. See #349