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Changes needed for https://bugs.launchpad.net/openquake/+bug/1101257

See https://docs.google.com/a/openquake.org/document/d/1vcyo921lJlgtj_X7JcQbWULmpq4AJGsLfqaHRMu5kYU/ for details

It also fixes https://bugs.launchpad.net/openquake/+bug/1096889

https://github.com/gem/oq-risklib/pull/31 it is required.