Arbor.js: introduce function childrenArray, which equals Object.keys(this.edges).

Enables introducing, later on, a caching function.
Arbor.js: the return value of childrenArray is now used like a generic, immutable array.

This serves two purposes:
 * speed ups: Array.reduce in particular is far slower than a plain for loop.
 * childrenArray is now replaceable by a function that returns an Uint32Array
when all nodes are numeric IDs.
SynapseClustering: speed up, and fix an error.

The error was at:
 if (delta_density[id] < 0) return;
... which now, being a loop instead of a forEach, the return
should have been a continue.

The speed up is based on using Arbor.allNeighbors() rather than allSuccessors()
which avoids having to generate the neighbors with an Array.clone operation
for every treenode.

Additionally partitions are now iterated with a for loop rather than a forEach,
which removes levels of closure lookup.
GraphPlot: fix error, asymmetry of inputs and outputs were switched.
SynapseClustering: speed up by not adding synapses beyond a maximum distance,
namely 3 * lambda (about 3 sigmas).
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.