Method to return a GKInstance instead of an Object when retrieving a value from an attribute on an instance

[jweiser]

A utility method in release-common-lib or an additional method in the GKInstance class in the Curator Tool repository is needed for retrieving an attribute value, which is a GKInstance object, as a GKInstance instead of a generic Object:

https://github.com/reactome/CuratorTool/blob/master/src/org/gk/model/GKInstance.java#L238

This will ensure casting the Object type to the GKInstance type in data-release-pipeline code is not needed.