Move the CheckerBoard class to verde.synthetic

[leouieda]

After #277 we won't be shipping sample data with Verde any more. This means that the CheckerBoard class will be alone in that module. We should move it to a verde.synthetic module instead, which could house other synthetic data generation methods. This should be done in 2 steps:

• Move the class to the new module but import it back into verde.datasets. Documentation should be updated to import from the new module. Issue a deprecation warning if it's used from there. For Verde 1.7.0 Release v1.7.0 #341
• Remove entire verde.datasets module along with the deprecation warnings (this is Remove the verde.datasets module #350). For Verde 2.0.0 Release v2.0.0 #331