Remove the hazard_calculation table

This is a huge change, involving the move of several methods of HazardCalculation into commonlib.
See https://bugs.launchpad.net/oq-engine/+bug/1371639.  A companion PR https://github.com/gem/oq-commonlib/pull/40 is needed. The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/692

PS: as a byproduct, https://bugs.launchpad.net/oq-engine/+bug/1267425 is fixed (i.e. intensity_measure_types
should not be specified when the intensity_measure_types_and_levels are specified, in the job.ini or from
the risk models).
Why the description sometimes is None?