Fix indexing of DimStacks with nested DimArrays

[tiemvanderdeure]

Solves #891

The logic here is that if getindex returns the eltype of a stack, then it should return a NamedTuple. This should be more robust then checking if any of the returned objects are DimArrays.

I've tried with a few DimStacks and don't see any difference in performance - but we might want to test a bit more rigorously?

[rafaqz]

That does seem more correct

What's up with the docs?

[tiemvanderdeure]

I don't know about the docs, but it seems unrelated because there aren't any stacks in those examples?

Also it's some trivial whitespace thing I think

[tiemvanderdeure]

I realized an alternative way to do this is just to test whether or not all(hasdim(D, dims(s))) && all(map(d -> d isa Dimension{<:Integer}, D)). That might be a bit easier on the compiler?

[rafaqz]

I think I like the current way better. But either way it would be nice to have a test of the edge cases this fixes