Solved the block size dependence in the risk calculators

See https://bugs.launchpad.net/oq-engine/+bug/1242372 and https://bugs.launchpad.net/oq-engine/+bug/1309371.
This branch requires the companion branch in risklib, https://github.com/gem/oq-risklib/pull/67

Tests passed here: https://ci.openquake.org/job/zdevel_oq-engine/377