Follow up of "Prototyping custom model/optimizer"

[guimarqu]

• remove subproblem Formulation

Currently, when the dev creates a custom model/optimizer, Coluna maintains a Formulation to handle subproblem variables. This formulation is useless because we can leave the subproblem variable management to the custom model and only maintains the subproblem representatives in the master.

Moreover, in column generation, we'll have to set the current cost of the representatives instead of the sp variables.