Add deprecation warning for VectorSpline2D

[leouieda]

Refer users to the implementation in Erizo instead
(https://github.com/fatiando/erizo). Remove the respective gallery
example since we don't want people using this class. In the Vector data
tutorial, point people to Erizo and include the deprecation warning.
See #205.

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.

[santisoler]

Seems good to me. I think the texts are sufficiently clear and show the differences between the uncoupled interpolation made by vd.Vector and coupled one that is applied by Erizo.