Disaggregation in event based risk calculator

Addresses https://bugs.launchpad.net/openquake/+bug/1160845 https://bugs.launchpad.net/openquake/+bug/1163200 and https://bugs.launchpad.net/openquake/+bug/1160993

It basically includes https://github.com/gem/oq-engine/pull/1109 and https://github.com/gem/oq-engine/pull/1111