Streamtree performance improvements (running list)

[jameshadfield]

The streamtrees implementation of #1902 seems fast enough, certainly for the v1 release, but there are a bunch of improvements we could make. In general, avoiding unnecessary calculations will be faster at the cost of ensuring they are recomputed when needed (e.g. we toggle off streamtrees). A running list:

• The branch thicknesses, node visibilities etc etc are still being calculated despite not being needed
• Similarly for some positining (d.depth, d.pDepth etc) properties, despite the nodes themselves not being rendered