[RFC] Refactor `ratio`

[glemaitre]

ratio is still a bit of a weird parameters. Here is a proposal for further development:

• remove the deprecation regarding float as ratio which is a valid parameter with binary target.
• 'all' in over-sampling should be changed to 'not majority'. This is currently doing the same process but this is weird for the user.
• cleaning methods should have their own ratio type. str should be accepted but dict should be replaced by a list since we cannot select how many sample to select. Also float would not be accepted for the exact same reason.

ratio should also be renamed. My proposal sampling_target?

[glemaitre]

@amueller @chkoar @jorisvandenbossche
Any remarks is welcomed.

[amueller]

I don't really know where and how this is used to give feedback, I think.

[massich]

are we talking about this ratio?

[glemaitre]

yep