Add explicit `NonInterpolatedDimension` ?

[rafaqz]

Currently the last N dimensions may not be interpolated. But it would be useful to allow arbitrary dimensions that won't be interpolated. The interpolator functor would then accept Int or : in those positions.

For context I'm looking at adding an extension in DimensionalData.jl, and due to the degree of abstraction it works at (dimension order can be treated as an implementation detail) we need to have the non-interpolating dimensions in arbitrary places rather than just the last dimension.

I guess this may complicate the algorithm to some extent, but I can PR if this sounds reasonable.