IterationUpdateCallback checking total cost decreased

Implementation of !384 (merged) should be further developed: returning non success message if total cost did not decrease.

added To Do (release) label

created merge request !386 (merged) to address this issue

mentioned in merge request !386 (merged)

added Doing label and removed To Do (release) label

closed via merge request !386 (merged)

mentioned in commit 33b20661