PatchesSelection: add the possibility to specify proportions/datasets custom names

At the moment, a user can use the mode chessboard to create 2 balanced datasets:

• "A" : 50% of points
• "B" : 50% of points

Maybe we could (like in decloud) enable to create any number of datasets, with any proportions, e.g.

• "train": 75% of points
• "valid": 15% of points
• "test" : 10% of points

That would imply to add a new strategy option, that would have at least 2 subparameters

• "names": names of the datasets
• "proportions": corresponding proportions of the datasets