Move plotting functionalities to a dedicated module

[rhugonnet]

Raised by @adehecq in #159

[rhugonnet]

Remark made in #218: For plots: it would be good to have a consistent approach at plotting and passing axes/grid objects to allow easy user edits without duplicating input parameters. But, realistically, this would have to be done in a later PR focusing on a plotting library. (skgstat plotting functionalitiers allow to pass ax, grid, and fig object back and forth, maybe we can check that out? or maybe there's better solutions out there, it's quite a common functionality people want to have)