Bug fix: Reset force coords when refitting splines

[leouieda]

Previously, the Spline set the force coordinates from the data
coordinates only the first time .fit was called. This means that when
fitting on different data, the spline would still use the old force
coordinates. Now, the spline will use the coordinates of the current
data passed to fit. This only affects cases where force_coords=None.
It's a slight change and only affects some of the scores for
cross-validation.

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.