Use functions instead of trait methods as the API for `src/spline.rs`

[DJMcNab]

I don't really think that this module has much advantage for understandability in this form. I'd rather see two functions, which can internally use a shared trait.

Originally posted by @DJMcNab in #2 (comment)

That module currently has a trait which is implemented for two types, including a (slice, slice, t) tuple, and a method which iterates through that type. I'd much rather see this be a function instead, so that the parameter names have meaning.