Extend graph visualization to give more flexibility in filtering

STATUS:DRAFT 

1. Choosing nodes:  
   * Users should be able to refer to cell sets and classes by rdfs:label.  
   * User should also be able to refer to cell sets by obs key + value.  
2. Iterpretation of list of input nodes:
   * Bottom up: Choose cell sets and show graph above these.
   * Top down choose classes and show graph below these.