WebGLApplication: new function Skeleton.splitByFlowCentrality
returning a map of node ID vs true for nodes that belong to the axon.
WebGLApplication: re-implement color connectors by axon and dendrite using Skeleton.splitByFlowCentrality.
WebGLApplication: new skeleton coloring scheme "Axon and dendrite"
using Skeleton.splitByFlowCentrality, with:

axon: green
dendrite: blue

... the same colors that coloring connectors by axon and dendrite uses.

This coloring scheme provides a visualization of the axon and dendrite domains
while retaining the pre/post identity of the synapses when colored normally
with red and cyan.