GroupGraph: new button to measure risk of an edge

The edge is either selected or both its input and output nodes are selected.
Autapses are labeled with maximum risk.

Uses the flow centrality method, computing the fraction of input
synapses removed when cutting the arbor at the nodes receiving
the synapses that make up the graph edge.

Skips groups.
Cannot yet handle split nodes.

Needs heavy refactoring.
GroupGraph: fix error in shift+alt+click to activate a skeleton.
Arbor.js: rewrote functon findBranchNodes, more performant and returns now an object
instead of an array.
Arbor.js: better docs for function findBranchAndEndNodes
Arbor.js: new function lowestCommonAncestor  of a set of nodes.
MorphologyPlot: adapt to new return value of Arbor.findBranchNodes
GroupGraph: store risk in the edge's data.