1. Res. Student, Univ. of Oxford, Dept. of Engrg. Sci., Parks Rd., Oxford, England OX1 3PJ.
2. Lect., Univ. of Oxford, Dept. of Engrg. Sci., Parks Rd., Oxford, England OX1 3PJ (corresponding author). E-mail: martin.williams@eng.ox.ac.uk
3. Lect., Univ. of Oxford, Dept. of Engrg. Sci., Parks Rd., Oxford, England OX1 3PJ.