variable node height

[ajo2995]

Currently, the node height is constant. If a zone needs more space to render content for a node it should be able to update the node's height. Conversely, you could shrink a set of nodes and they would be dealt with accordingly by each zone. Maybe a node height zone could let users change the height of a node by dragging the bottom edge up/down.

The genetrees reducer should manage the top coordinate and height of each visible node as well as the total height of the display. That way, individual zones won't have to figure out each node's location