Returns bounds on the optimisation variables.

def _var_bounds(self):
        """ Returns bounds on the optimisation variables.
        """
        x0 = array([])
        xmin = array([])
        xmax = array([])

        for var in self.om.vars:
            x0 = r_[x0, var.v0]
            xmin = r_[xmin, var.vl]
            xmax = r_[xmax, var.vu]

        return x0, xmin, xmax