This is a pre-copyedited, author-produced PDF of an article accepted for publication in Journal of Physics A: Mathematical and Theoretical following peer review. The version of record is available http://iopscience.iop.org/article/10.1088/1751-8121/50/5/055205/meta;jsessionid=48745C2AEC73734D29478CE66DC3286E.c2.iopscience.cld.iop.org