Add iteration to the protocol model.  If the children_relationship of a
protocol is set to anything other than the N/A relationship, the
children of the parent job's product, according to this relationship,
will be used to create new jobs using these children as the prodcuct of
these new jobs.  If the children_relationship is the N/A relationship,
the previous rules apply.

Added test for this, and reenabled the job chronology test.