Note for re-write: SliceRepresentation exposes I/J/K/X/Y/Z slicing properties

[jadh4v]

          SliceRepresentation exposes I/J/K/X/Y/Z slicing properties. I wonder if we should split out the functionality into something like a `SliceStackRepresentation`. What do you think @jadh4v @sankhesh?

A note for the rewrite: depending on how the types work out, I may split the ImageArrayMapper and ImageMapper into two separate components, but I haven't given it much thought just yet.

Originally posted by @floryst in #95 (review)