tokens: The authors are grateful to the anonymous referee for constructive comments and suggestions that have improved this paper. Ch.H. and C.R.S. are grateful for the financial support of the European Community under the FP7 by an ERC starting grant number 257431. D.A.D. is grateful for funding from the UK Science and Technology Funding Council via grant number ST/I001808/1. It is a pleasure to acknowledge stimulating discussions with Drs H. E. Potts and P. B. Rimmer. Further to this, the authors would like to thank D. Juncher for discussions regarding dust opacity. The authors would like to acknowledge our local IT support.
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