Allow zero degree polynomials in Trend (a mean)

[leouieda]

There was no real reason to not allow them in the past. A zero degree
polynomial is a mean. This can be useful to test if a simple static
shift in the data is enough of a trend.

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.