Crystal Structure and Magnetic Behaviour of the New Gadolinium Carboxylates Gd2(ClF2CCOO)6(hypy)2, Gd2(F3CCOO)6(hypy)2, Gd2(F2HCCOO)6(hypy)2 and Gd2(Cl2HCCOO)6(H2O)2(hypy)2