Managing state variables

Need a way to either 1. show multiple OR 2. prioritize.

Use case:  An entity pool node with two or more state variables. One or more may be useful. However, which one is shown seems to depend on the order in which the source SBGN-ML is processed.  

Related to https://github.com/PathwayCommons/pathways-search/issues/98 ; https://github.com/PathwayCommons/app-ui/issues/87