Fix misleading error message for coordinate validation

[meredithdoan]

The error message for Monotonicity(usize) claims that coordinates must be "sorted and non-repeating," but the actual implementation only checks for monotonically increasing values (using <=), which allows duplicate/repeating values. This makes the error message deceptive - it promises stricter validation than is actually performed.

An alternative fix would be to change the implementation to use < instead of <=, enforcing strictly increasing coordinates (no duplicates). See https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.PchipInterpolator.html: SciPy's interpolation functions explicitly reject duplicate values as they would make the function "overspecified." However, changing the implementation to require strictly increasing coordinates would be a breaking change and may affect existing users. This PR opts for the minimal fix of correcting the error message to match the current behavior.

[kylecarow]

Thanks for the PR @meredithdoan! I'll go ahead and merge this.
Good catch on this; I had forgotten to update the error message when I loosened that restriction.

If anyone in the future has feelings about changing the comparison to be < rather than <=, I'm open to making a new major version to do so.

[kylecarow]

@meredithdoan I've published these changes in v0.8.2!