Port some data container functionality out of FastAI.jl

[lorenzoh]

FastAI.jl currently has some data container functionality that I've found very useful. On the last ML ecosystem call, I and @darsnack + @ToucheSir discussed that it makes sense to have some of that in MLUtils.jl. The relevant FastAI.jl code can be found here: transformations.jl

Specifically, there are some data container transformations that I believe should be ported:

• mapobs(f, data), a lazy map over any data container. Generally useful
• groupobs(f, data), returns a Dict with keys return values of f(obs) and values a datasubset of obss that returned the same f(obs). Not sure if Dict is the right type here, but NamedTuple is too restrictive. Useful for example to create train/test splits based on some value in each observation.
• filterobs(f, data), does what you'd expect, returning a datasubset
• joinobs(datas...) treats multiple data containers as a single one. Open to a better name for this.

There are also some data container primitives for working with tables and files, but let's put that into another issue.