Adds a constraint to the model.

def add_constraint(self, con):
        """ Adds a constraint to the model.
        """
        if isinstance(con, LinearConstraint):
            N, M = con.A.shape
            if con.name in [c.name for c in self.lin_constraints]:
                logger.error("Constraint set named '%s' already exists."
                             % con.name)
                return False
            else:
                con.i1 = self.lin_N# + 1
                con.iN = self.lin_N + N - 1

                nv = 0
                for vs in con.vs:
                    nv = nv + self.get_var_N(vs)
                if M != nv:
                    logger.error("Number of columns of A does not match number"
                        " of variables, A is %d x %d, nv = %d", N, M, nv)
                self.lin_constraints.append(con)
        elif isinstance(con, NonLinearConstraint):
            N = con.N
            if con.name in [c.name for c in self.nln_constraints]:
                logger.error("Constraint set named '%s' already exists."
                             % con.name)
                return False
            else:
                con.i1 = self.nln_N# + 1
                con.iN = self.nln_N + N
                self.nln_constraints.append(con)
        else:
            raise ValueError

        return True