GroupGraph: refactored risk measurement.

Also interpreting 0 as MIN and 1 as MAX, in edge labels.
GroupGraph: include number of outputs in the measure of synapses left out
when pruning the tree at the lowest common ancestor to compute
the synaptic risk.
New URL compact-arbor

Similar to compact-skeleton, but the connectors (json[1]) contain
the complete chain:

treenode1 -> confidence1 -> connector -> confidence2 -> treenode2 -> skeleton2
utils.js: new function ArborParser to parse JSON from both compact-skeleton and compact-arbor
Measurements table now uses compact-arbor and counts both output sites and output synapses,
which are different numbers when synapses are polyadic.
CircuitGraphPlot: fix error when no neurons are connected.
utils.js: fix incorrect documentation of ArborParser.