Arbor.js: renamed subArbors to connectedFractions, which is far less confusing
given that subArbor (singular) exists and does something completely different.
Arbor.js: new function minDistancesFromTo which finds the minimal distance
from a node in sources to a node in targets.

For example to find the distances from nodes hosting a synapse to nodes with a mitochondrium.
AnalyzeArbor: first attempt at histograms of distances from a synapse to the nearest mitochondrium.