RFC Reorganisation of the module

[glemaitre]

under-sampling starts to be crowded. I think that we could actually reorganise the module using the semantic:

• prototype selection;
• prototype generation;
• over-sampling

We could create prototype selection/generation folders under under-sampling and allow for import at the top level --- e.g., from imblearn.under_sampling import CNN. Like this it will not change anything for the user but will make more clear the documentation.

@chkoar WDYT?

[chkoar]

I wouldn't go deeper than two levels. But If I had to choose from these two options I would vote for the latter.