Option to treat voltage magnitude bounds as soft constraints

[bcdandurand]

I'm noting this issue for now, though I'm not working on it right away. This issue builds on some recent thoughts of @kibaekkim regarding the reformulation of the voltage magnitude bounds as soft constraints.

For SDP forms, there could be merit in providing a utility for reforming the voltage magnitude bound constraints as soft constraints with exact penalty terms for solution approaches that relax dual PSD constraints. When the full PSD cone in the primal problem is feasible, particularly the origin, due to the soft constraint reformulation, this could translate to favorable boundedness properties for the dual problem. I'll think about this more later.