Loss per event per asset

Minimal changes to support the feature. See https://bugs.launchpad.net/oq-engine/+bug/1383291
The real meat is in the engine. The tests are green: https://ci.openquake.org/job/zdevel_oq-commonlib/153/
Yes, sites_disagg is optional, so a missing sites_disagg is valid. I am going to improve the docstring.