TOPOLOGY OF $U(2, 1)$ REPRESENTATION SPACESPartially supported by the Fundação para a Ciência e a Tecnologia (Portugal) through the Centro de Matemática da Universidade do Porto, and by the European Commission through the Research Training Networks EAGER and EDGE (Contracts Nos HPRN-CT-2000-00099 and HPRN-CT-2000-00101)