* To appear in Information and Inference: A Journal of the IMA. Made
following changes after review process: (a) Corrected typos throughout the
text. (b) Corrected choice of sampling distribution in Section 5, see eqs.
(5.2), (5.3). (c) More detailed comparison with existing work in Section 8.
(d) Added Section B in appendix on roots of cubic equation Access Model/Code and Paper