allow creation of model (transformer) By modality

[LionelMassoulard]

Idee : facilite the creation of a model that actually fit one model per type of modality.
Example : fit on model per country if a country.

The code could look like that :
SplitByVariable(model, variable)

Under the hood.

• during the fit the data will be splitted by the different value of 'variable', one model per variable will be fitted
• during the pred : apply the corresponding model