Selects the worst plan of a person (most likely for removal), but respects
the set plan types in a way the no plan is selected that is the last one of
its type.
(I would say that it can select the last of its type if it finds nothing else. However, this algo should only
be used if an agent has more plans than maxPlansPerAgent, so make sure that that parameter is set large enough for
your purposes. kai, oct'09)
Plans without a score are seen as worst and selected accordingly.