Quick access Dimension indices

I find it would be useful to be able to quickly access the indices of a DimArray as a `Vector` rather than the DimensionalData object returned by `dims()`. I suppose this could be a simple method, something like the following? ```julia function getdim(D::AbstractDimArray, args...) collect(dims(D, args...).val) end ``` Apologies if this is covered elsewhere. I'm happy to contribute a PR if this would be useful.