Fix Insured losses computation

Currently, when you compute insured losses on a set of loss ratios the returned output changes the order of the values. This is a problem if the loss ratios correspond with  different ground motion values and you want to compute aggregate insured losses over different assets. This pull request addresses this problem.

In order to have a clean diff, https://github.com/gem/oq-risklib/pull/35 should be merged first.