Arbor.js: radialDensity function.
Remove obsolete function createCytoscapeGraphWindow.
utils.js: define SkeletonSource.prototype.annotate_skeleton_list

Will avoid reimplementing it for each SkeletonSource.
utils.js: define fetchCompactSkeletons

Fetches each skeleton given an array of IDs, one at a time to avoid time outs
and blocking a server thread for too long.
Dispatches returned JSON and failures and done events via handler functions.

It's a refactoring of a function previously in the webglapp.js,
for use in other widgets.
Arbor.js: fix computation of sholl.
Arbor.js: new function nextBranchNode
Arbor.js: new function flowCentrality

to measure e.g. centrality of a node by counting the number of paths
from all input synapses to all output synapses that pass through the node.
overlay.js: new function SkeletonAnnotations.getActiveNodeVector3
Morphology Analysis: Sholl analysis and more.

WIP