1. IMAR, Department of Exact and Technological Sciences of the Portuguese Distance Learning University, R. Escola Politecnica 147, 1269-001 Lisbon, Portugal, Biomedware, Inc., 516 North State Street, Ann Arbor, Michigan 48104-1236, ISEGI/CEGI, Institute for Statistics and Information Management of the New University of Lisbon, 1070-124 Lisbon, Portugal, and IMAR, Faculty of Science and Technology of the New University of Lisbon, 2829-516 Caparica, Portugal