Facile Syntheses of Selenium- and Tellurium-Containing Metal Cubanes, [Cp*M(.mu.3-E)]4 (Cp* = C5Me5; M = Rh, Ir, Ga; E = Se, Te), and X-ray Crystal Structures of (Cp*RhSe)4, (Cp*IrSe)4, (Cp*RhTe)4, (Cp*IrTe)4, and (Cp*GaTe)4