Add methods for mean, sum etc on AbstractDimensionalArray

[rafaqz]

These methods should call themselves on parent, and rebuild the dims manually where necessary - its not clear if relying on similar is fully workable.

It's also not clear if they should be combined with the current methods, or dimensional indexing on AbstractArray and regular indexing on AbstractDimensionalArray should stay separate.

Currently dims2indices conversion and rebuild() are in different methods.