+Postal address: School of Engineering and Physics, University of Dundee, Dundee DD1 4HN, Scotland
++Present address: Fundação Oswaldo Cruz, Far Manguinhos, Rua Sizenando Nabuco, 100 Manguinhos, 21041250 Rio de Janeiro, RJ Brazil.
1Supplementary data for this paper are available from the IUCr electronic archives (Reference: NA5018 ). Services for accessing these data are described at the back of the journal.