Remove the dependency from the gmf/gmf_set tables in the XML export procedure

Now the export procedure reads directly from the gmf_agg table, so that the gmf and gmf_set tables can be purged and we can save a lot of disk space. This also helps with https://bugs.launchpad.net/oq-engine/+bug/1169078, since now the export procedure is faster (3-4 times faster in some simple example I tried). However the memory occupation is still big and the major problem of the XML export remains unsolved.