Allow grouping of nodes of the same type

[andrewsu]

This starts getting in to heavier UI development, so we should discuss the game plan before committing to implementing this ticket. But, imagine the scenario below:

currently I can add all 14 of those WP pathways to the network, but they show up as separate nodes. Suppose I want to get all genes associated with any of those pathways. I would want to aggregate them into a single node. If I click that node, it should show me services that take that node type. If I click a service, then the explorer should execute 14 calls for each of those pathways (or even better, a single query that combines those 14 pathways).