Revise insured losses algorithm

Addresses https://bugs.launchpad.net/oq-risklib/+bug/1217438

Requires https://github.com/gem/oq-risklib/pull/60

As now the signature of the Classical workflow ```__call__``` is equal to the corresponding ones in the other workflows, I refactored some logic from the ```__call__``` method of the workflow classes into the ```CalculationUnit``` class. As result, workflows are decoupled from getters and monitors and they don't use side effect to compute statistics.