Technetium bis(.mu.-oxo) dimers of 1,4,7-triazacyclonane-N,N',N''-triacetate (TCTA). Synthesis and characterization of [(TCTA)Tc(.mu.-O)2Tc(TCTA)]n- (n = 2, 3) and the crystal structure of Ba2[(TCTA)Tc(.mu.-O)2Tc(TCTA)](ClO4).9H2O