Arbor clustering: perform less operations for the same effect.
Arbor.js: new functions neighbors(node) and allNeighbors()
Arbor.js: new function downstreamAmount, returns a map of node vs
amount of arbor downstream of each node.

The "amount" is computed by a function that takes two arguments: parent and child nodes.
Merge branch 'master' of github.com:acardona/CATMAID