Expand / Collapse Support

[Nithos]

Allow for dynamic expand/collapse interaction of nodes that are in a hierarchical grouping. This is an alternate mode to zoom as the viewport should automatically change to fill to the full visible graph.

Some decisions and unknowns at the moment

1. is the re-layout done on the fly within grafer? or externally?
2. can we support animated transition between the two
3. what are the performance considerations and possible issues