Split package into multiple and make this a Meta package

[Evizero]

This package has come a long way, but at the moment it feels a bit like the package equivalent of a blob design. I am very happy with MLLabelUtils because it is a single concern kinda package, which makes it easier (and more fun) to design and flesh out.

I think it would make sense to make this package a convenience collection package that reexports data related ML packages.

• [x] MLDataPattern.jl (done). Only deals with the data access pattern, which are currently the dominating part of this package.

• [x] MLDatasets.jl. Self explanatory.

• [x] MLLabelUtils.jl. All things concerning classification labels.

• [ ] MLDataGenerators.jl (doesn't exist yet). Move artifical data generators there. Also absorb #9

• [ ] MLPreprocessing.jl (in progress). This aspect is a bit neglected so far.

Last but not least it could offer some glue code. For example if MLDataUtils is the meta package it could define the needed interface for DataFrame and friends, so they just work out of the box.

[Evizero]

parts of the current documentation should probably be moved to the JuliaML website instead

[Evizero]

As a side note, I will remove precompilation from MLDataUtils, once the code move to MLDataPattern is complete. This is because I will add a Requires.jl dependency here, so that we can have default support for all kinds of different data sources, such as DataFrame and DataTable