Add a tag to the ruptures

See https://bugs.launchpad.net/oq-engine/+bug/1223362
The tag attribute is essential to have a reproducible ordering of the ruptures (passing the same ruptures in a different order generates different GMFs). The tag is a string built from the logic tree realization ordinal, the SES ordinal, the source id (the one in the model, not the db)  and the ordinal number of the rupture. There are no references to database serial numbers. See the companion PRs: https://github.com/gem/oq-nrmllib/pull/109 and https://github.com/gem/oq-hazardlib/pull/135