Arbor.js: new function simplify, to create topologically accurate
but simpler versions of the arbor, preserving a given set of nodes
as well as any relevant branch nodes in between.
SynapseClustering: new function to retrieve clusters as maps rather than arrays.
GroupGraph: new button to split by synapse clustering.

Now one or more selected nodes can be given a specific bandwidth value.

Splitting occurs in the client.