Added creatorTypeID to the PK in itemCreators, because a creator could conceivably have two roles on an item
Throw an error if attempt to save() an item with two identical creator/creatorTypeID combinations, since, assuming this shouldn't be allowed (i.e. we don't have a four-column PK), there's really no way to handle this elegantly on my end -- interface code can use new method Item.creatorExists(firstName, lastName, creatorTypeID, skipIndex), with skipIndex set to the current index, to make sure this isn't done
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