Gmf from ruptures

This PR implements a risk calculator able to compute the event based risk outputs directly from the stochastic event sets, by recomputing the GMFs on-the-fly (https://bugs.launchpad.net/oq-engine/+bug/1349757). The performance is not yet optimal and the calculator should be considered in experimental stage. Future work will change and improve it.

The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/486
Unfortunately the suffix _lt is used several times, for instance, in the name of the files (i.e. in the QA tests we have things like gsim_logic_tree_file = haz_map_gsim_lt.xml). Changing the suffix to a suffix would break a long standing tradition without any real advantage. I would advocate the status quo.