util function to change the resolution of a spline

[ddudt]

Splines don't have a proper basis class, but it would be helpful to have a function to effectively change their "resolution". Something like: new_values, new_knots = change_spline_resolution(old_values, old_knots, method)

This is effectively already done in OmnigenousField.change_resolution but should be a standalone function that can be used elsewhere.

[dpanici]

resample instead of change_resolution