Expose the optimal spline in SplineCV

[leouieda]

SplineCV was keeping the best Spline instance in SplineCV._best which
goes against the scikit-learn standard of storing estimated parameters
in variables ending in _. Change that to SplineCV.spline_ and add it
to the class attribute list.

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.