Support for the simplification of the risk calculators

See https://bugs.launchpad.net/oq-engine/+bug/1376157. Requires https://github.com/gem/oq-risklib/pull/79
The tests are green: https://ci.openquake.org/job/zdevel_oq-commonlib/155
It is useful, because it tests that a warning is printed when an unknown parameter is given