CircuitGraphAnalysis: validate adjacency matrices prior to trying to compute eigenvectors.

Matrices that fail:
  * zero edges
  * only self-edges (autapses)
Arbor.js: fix corner case in measurement of terminalCableLength
WebGLApplication: new function Skeleton.getPositions
Arbor.js: fix error in function clone.

The edges of the clone were not created properly.
Became evident when computing the flow centrality of an arbor
with a branching root, which needs to be rerooted, which needs
to clone the arbor.
CircuitGraphPlot: remove unnecessary check.