bring back grouping in trackvis

[NateLanza]

Nodes should be groupable. A group of nodes is collapsible in the visualization. Ephemeral nodes should automatically collapse together (once re-implemented in Trrack/trrackjs#60) and users should be able to manually select groups of nodes to collapse (labels for these collapsed groups would be nice). Grouping should probably only work within single branches and direct linear relationships. However, branches need to be able to start from the middle of groups. Manual grouping will require a multiselect action in Trrack, which should at a minimum support shift clicking to select linear groups down a branch. Multiselecting probably shouldn't update the state of the vis, leaving it at the state of the last single selected node.

[kirangadhave]

Groups should support annotations — treating groups as nodes

[NateLanza]

This also needs a linked issue in trrack, as the groups should be stored by the trrack object.