Code simplifications: listing calls

[janvanrijn]

Currently, we have:

• openml.utils._list_all
• openml.*.list_* (where * can be datasets, flows, tasks, setups and runs)
• openml.*._list_*
• openml.*.__list_*

Making a grand total of 16 functions, for what is basically a single function. Seems like a lot of confusion to me. Would be great if we could look for ways to consolidate this to at most 6 different functions.

[mfeurer]

I agree that we should try to simplify this.

I'm just posting the outcomes of our offline discussion for reference: If there was a machine readable version of the API docs, the list function could request it via an API call and check the arguments provided by the user based on this. This would allows to remove manual checks at the cost of very minor overhead.

[eddiebergman]

@janvanrijn I see the code has at least one core _list_all. Is this issue still relevant, as in you'd like to remove the def list_X function?