Added Multiplicative Costing
Added Multiplicative Costing

Added a flag that can be used in expdesigner to opt for setting the
experimental cost to multiply, rather than add to, the bayes risk.  This
is appropriate in cases where the difference in the magnitude between
the bayes risk and experimentally relevant costs are significant.

Alan-Robertson/python-qinfer,ihincks/python-qinfer,whitewhim2718/python-qinfer,QInfer/python-qinfer,MichalKononenko/python-qinfer,csferrie/python-qinfer