Allow to add box constraints by name

[brunomorampc]

At the moment the user must provide all upper bounds and lower bounds of all variables, only if just one variable is constrained.
Allow to specify the bounds of specific variables by name. Something like

mpc.set_box_constraints(names=['vx', 'vy'], ub = [10, 7] , lb = [-10, -7])

Side notes

This would decrease the computational burthen of approaches e.g. SMPC