Evaluate a splined radial basis

[sirmarcel]

Hello, as discussed on Slack, here's a feature request: It'd be nice to be able to evaluate a (splined) radial basis at arbitrary points for testing purposes. This would allow fine-grained testing of alternative implementations, as opposed to end-to-end testing via the SphericalExpansion calculators.

[Luthaf]

This would mainly require copying the code from here to a Python function:

https://github.com/Luthaf/rascaline/blob/5ac2ee9e6b45f31ba608c02d6d8dd7d82b2009a9/rascaline/src/math/splines.rs#L228-L284