Allow for hiding of root node

[dvzacharycutler]

No description provided.

[kirangadhave]

Should this be handled in trrack-vis as well as trrackjs?
If the visualization hides the root node, should undo using trrack API go to the hidden root node?
How would trrack-vis handle this state?

[dvmartinweigl]

We were able to hide the root node in our application (sightline). As @dvzacharycutler suggested we removed the root node from trrack.graph.backend.nodes and set the root to the first child node. Of course we had to ensure that the node navigation stays within our customized graph. Even though this solution basically works, it would still be nice to support a custom root node by default in TrrackVis.