SynapseClustering: method to find the axon by centrifugal synapse flow centrality.
Unified method to find the axon across WebGLApplication and GroupGraph.

Before, the GroupGraph was using the max value of the plateau of
centrifugal synapse flow centrality values rather than the threshold
method. The latter is far more robust to small arbor differences
and therefore the preferred method throughout.
Merge branch 'master' of github.com:acardona/CATMAID