GroupGraph: fix error in counting members of a group, for the label.
GroupGraph: fix error. When creating a new group that includes an existing one,
the existing one has to be removed.
utils.js: ArborParser.cache to cache the result of Arbor functions.
CircuitGraphPlot: cache the result of several Arbor functions
that are invoked multiple times in loadAnatomy.

This is safe because:
1) The Arbor's content does not change after the call to cache.
2) The values of the cache are never modified by Arbor functions.

Results in significant speed ups.
CircuitGraphPlot: replace cache'd nodesArray with a native Uint32Array.

Results in a minor speed up for loadAnatomy.
CircuitGraphPlot: approximate segregation index based on splitting the arbor
at the maximum flow centrality nodes.

Parameter-free, and much faster than using SynapseClustering with
density hill map etc. The results are good enough for classification.
WebGLApplication: new function Skeleton.createPrePostCounts
WebGLApplication: flow-centrality shading uses createPrePostCounts
WebGLApplication: new synapse coloring mode "arbor-and-dendrite"

Splits the arbor at the max flow centrality nodes and colors
one cluster in green (the most presynaptic) and the other in blue.