Correction to the disaggregation equation

Addresses: https://bugs.launchpad.net/openquake/+bug/1116262

Before this is merged, we'll need to run oq-engine disagg tests and update them accordingly, since the results will be different.

We'll also need to update oq-engine to use `disaggregation_poissonian`, instead of `disaggregation`. The latter is now deprecated. Update: Those changes are implemented here -> https://github.com/gem/oq-engine/pull/1044.