TabulatedModel x fitting data that works for single property or property group

[ConnorPigg]

The tabulated model should be able to accept x values to be used to fit either a single property or a composite model for multiple properties. The problem is that there is not enough information to distinguish if the x is a feature_domain of a set of properties or simply a set of x. If it is a feature_domain, how to identify the slice to use as the x for interpolation? This problem does not occur for the LinearModel as the model is independent of x. Further, LinearModel requires prop.x to be a subset of x which covers both the case of a single property or a property group. Need a stronger definition of the interface of tabulated model.

[jmborr]

Good issue. How about a TabulatedCompositeModel ?