Replace outdated `Rbf` with `RBFInterpolator`

[NiklasVin]

The class Rbf that is used in expression_core.py is according to the documentation outdated and no longer maintained.
At some point it could make sense to replace the Rbf with RBFInterpolator.

[BenjaminRodenberg]

I would suggest to take care of this as soon as #28 is merged and we can consider this adapter stable.