Department of Statistics, Macquarie University, North Ryde, NSW 2109, Australia. peter.humburg@well.ox.ac.uk|Department of Statistics, Macquarie University, North Ryde, NSW 2109, Australia. peter.humburg@well.ox.ac.uk|Department of Statistics, Macquarie University, North Ryde, NSW 2109, Australia. peter.humburg@well.ox.ac.uk|Department of Statistics, Macquarie University, North Ryde, NSW 2109, Australia. peter.humburg@well.ox.ac.uk