Once a task is marked as dynamic its duration is set as the sum of its
children's durations. Previously the durations were all converted to hours;
now the task's duration type is changed to the common duration type of
the children, if there's one, and its duration is recalculated in that unit.
If there's no common unit (ones in hours, others in days) then do as before
but set unit to hours (previously it kept the unit and did some math).
Fix to allow clearing dependencies in the task edit form.