* Issues was that ReferenceItem was not calling resolve when doing a find
* Instead, this change uses the item's interators (which do call resolve) in the find methods
* Implemented a work around that caused issues in ModelBuilder when unloading a model resource whose components where being referenced in an attribute resource.  Prior to this fix, the associated components were no longer being displayed.
* Changed the constriant test to check that this fixes this issue
* Fixed an issue with DiscreteModel's RemoveModel Operator - it needed to have it's input model held by the operation's specification association.

See Issue #244