1. CERFACS, 42 avenue Gaspard Coriolis, 31057 Toulouse Cedex, France
2. ENSEEIHT, 2 rue Camichel, 31071 Toulouse Cedex, France
3. CNES, 18 Avenue Edouard Belin, 31401 Toulouse Cedex, France
4. Dept. of Mathematical Sciences, The University of Colorado at Denver and Health Sciences Center, Downtown Denver Campus, Denver, Colorado 80217-3364, USA