Update calls to risklib to the latest interface

As https://bugs.launchpad.net/oq-risklib/+bug/1174301 the returned values of risklib calls are just tuples of numpy arrays. So, every call to the risklib calculators needs to be updated.

This is the companion of https://github.com/gem/oq-risklib/pull/53

In order to avoid future conflicts it also requires https://github.com/gem/oq-engine/pull/1152