Compute ground motion values from Stochastic Event Set in a risk calculation

Addresses https://bugs.launchpad.net/oq-engine/+bug/1205623

It needs https://github.com/gem/oq-hazardlib/pull/129 and https://github.com/gem/oq-engine/pull/1236

Build status: http://ci.openquake.org/job/zdevel_oq-engine/91/