  This version corrects the first one in eq.3.4 and table 1 where there was a
bar missing in the 126 representation. All other equations and results are
unchanged and correct. However, the criticism we had made in the earlier
version to the work of Caldwell and Mohapatra was not correct and has been
removed from the "note added" where it appeared. Moreover one new reference has
been added to ref.13.
