(1) IFT-UNESP Sao Paulo Brazil e-mail: romello@ift.unesp.br, (2) DFMA-USP Sao Paulo Brazil e-mail: rivelles@fma.if.usp.br, 12 pages, RevTex 4, letter paper, PACS-Code: 02.20.Qs, 02.40.Yy, 03.65.Fd, 04.60.Kz, 11.30.Cp, Revised version of paper published in J. Math. Phys. 45, 1156 (2004).