Huge geometries

Branch "standardization" should be merged in master first.

Addressing https://bugs.launchpad.net/oq-svir/+bug/1253023 and https://bugs.launchpad.net/oq-svir/+bug/1253019

Using SAGA QGis plugin to cope with complex geometries. It works properly in case SAGA is correctly installed. I tested it with the "whole america" example, which on my laptop took something like one minute (instead of "forever").
We still need to find out how to successfully check if the SAGA plugin is properly installed and usable. In case it is not, we would like to use our own approach to aggregate losses by zone. The checks suggested by Marco, unfortunately, are not working as expected.