Returns the contents of the case to be used in the OPF.

def _unpack_case(self, case):
        """ Returns the contents of the case to be used in the OPF.
        """
        base_mva = case.base_mva
        b = case.connected_buses
        l = case.online_branches
        g = case.online_generators
        nb = len(b)
        nl = len(l)
        ng = len(g)

        return b, l, g, nb, nl, ng, base_mva