QGIS plugin aggregating loss data by region and joining it with social vulnerability data already aggregated by region

Addresses https://bugs.launchpad.net/oq-svir/+bug/1224401
The possibility to "purge" empty regions containing no loss points is currently commented out. We need to further discuss what is the desired behavior in case we have a social vulnerability index for a region containing no loss points.

Addresses https://bugs.launchpad.net/oq-svir/+bug/1229193
A layer containing social vulnerability index and risk attributes is produced, and simple indices combining social vulnerability index and risk are calculated and written to the same layer as new attributes. The user can take advantage of built-in QGIS functionalities to make new custom calculations using the existing attributes.

Still, standardization of loss data needs to be implemented (possibly giving the user a choice on the standardization algorithm), in order to build a risk index that can be meaningfully compared with the social vulnerability index.