remove ObsDims and data iterators

[CarloLucibello]

Continuation of #44
(I'm a bit lazy to figure out how to push into another person PR right now so I'm opening this new PR)

Compared to #44, this

• doesn't add the label-related functions
• fixes tests,
• improves some docs
• removes a few abstract classes for data iteration that I think we can well live without

[darsnack]

Thanks for taking this up. #44 is done; the only remaining blocker is JuliaML/MLDataPattern.jl#50. MLLabelUtils.jl PR (JuliaML/MLLabelUtils.jl#44) is also done.

Label-related functions were added cause that's what the comments for MLLabelUtils.jl indicated needed to be done. I'll give you write access to all my forks if you want to push the 3 PRs forward.

[CarloLucibello]

I'm merging this as it is, since downstream packages updates should not be considered blockers, they will be ready to update their compat bounds when they will be ready.

Also, I'd like to leverage getobs and nobs as soon as possible to finally have a consistent dataset interface across the ecosystem.

[darsnack]

I don't think everything here should have been merged. The state of #44 was necessary for the downstream PRs.

[darsnack]

The deleted types are also break MLDataPattern.jl. Can you revert this commit and just apply the version bump + test fix?

I agree with getting rid of types we don't need in the long term, but right now I am trying to update MLDataPattern.jl and MLLabelUtils.jl for the new interface. Too many clean up changes make refactoring the codebase too uncertain.

[CarloLucibello]

ok

[darsnack]

Thanks

[johnnychen94]

My main focus is still on JuliaImages so sorry that I don't have more spare time to help here.