Aggregate losses by zone id when possible

https://github.com/gem/qt-experiments/pull/8 should be merged first.

Addressing https://bugs.launchpad.net/oq-svir/+bug/1253007
In case we know in which zone each of the loss points is, we can simply aggregate losses by zone id, instead of performing a geo-spatial search.

This also fixes a bug in the process of joining SVI data with aggregated losses (now it correctly deletes from the final layer the features that don't have both SVI and aggregated loss)