Allow multiple `u` for the same interpolator

[rafaqz]

In many contexts many arrays need to be indexed at the same time with the same stencil and weights.

To do this we need an inner object that holds just the dims and cache, and and indexing object that can apply a precalculated stencil and weights to multiple arrays.

This would allowing interpolating into a mulitilayer DimensionalData.jl DimStack with only one stencil/weight calculation, and other user applications like that.

It could even be like a CartesianIndex object you can use in getindex.

[SouthEndMusic]

Wouldn't it be easiest to (lazily) concatenate the multiple u?

[rafaqz]

Different array eltypes are very common on Julia ;)

Also feels a little hands-on as a workflow.