Returns the problem dimensions.

def _dimension_data(self, buses, branches, generators):
        """ Returns the problem dimensions.
        """
        ipol = [i for i, g in enumerate(generators)
                if g.pcost_model == POLYNOMIAL]
        ipwl = [i for i, g in enumerate(generators)
                if g.pcost_model == PW_LINEAR]
        nb = len(buses)
        nl = len(branches)
        # Number of general cost vars, w.
        nw = self.om.cost_N
        # Number of piece-wise linear costs.
        if "y" in [v.name for v in self.om.vars]:
            ny = self.om.get_var_N("y")
        else:
            ny = 0
        # Total number of control variables of all types.
        nxyz = self.om.var_N

        return ipol, ipwl, nb, nl, nw, ny, nxyz