Simplified the risk calculators

See https://bugs.launchpad.net/oq-engine/+bug/1376157. Requires https://github.com/gem/oq-commonlib/pull/46 and https://github.com/gem/oq-risklib/pull/79.
Tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/774
No, the name a1, a2 and a3 are in the exposure file, so it would be confusing to rename them.