Arbor.js: terminalCableLength also returns the number of branches.
Arbor.js: new function pruneBareTerminalSegments
CircuitGraphPlot: fix error in using the neuronNameService.
CircuitGraphPlot: added number of branches to anatomy.
CircuitGraphPlot: add only PC 1 and PC2 to the menus, and later add as many
PC entries as necessary to add up to over 98% of all variance.
CircuitGraphPlot: add option to prune away empty terminal branches.

Many of these branches are the product of limitations in skeleton tracing
when doing so over serial sections, and may be labeled as "not a branch".
Synapse clustering: attempt at speeding up critical parts.
Arbor.js: speed up findEndNodes by 8x
Arbor.js: speed up allSuccessors by 4x
Arbor.js: speed up findBranchAndEndNodes, and adjust other functions
to slight change of API, which accelerates other functions by
not having to create a map of branch nodes from the array of branch nodes
(which was never used directly as an array except for counting).

Returns now:
 {ends: array of end nodes,
  branches: map of branch node vs count of successors,
  n_branches: number of branching nodes}
Arbor.js: speed up partition function.

From higher than O(6n) to about O(4n).
Arbor.js: fix typo in docs
Arbor.js: Strahler analysis is now correct.
Arbor.js: smoothPositions can now take an optional parameter for the initial value
to accumulate on.

This enables e.g. accumulating on an array rather than on an object,
given the identical syntax.
Arbor.js: enable distanceToUpstreamNodeIn to use arrays as well as objects
for the stops.
Arbor.js: fixed issues in terminalCableLength.
Synapse clustering: use a more lightweight synapse map.
CircuitGraphPlot: options to switch on/off each parameter for PCA.

Also added new measurements:
 * length of the hillock cable (very powerful), estimated from the flow centrality.
 * normalized histograms of asymmetries.
 * normalized cable length of terminal segments

Also removed some redundant operations, like the creation of synapse maps
for computing input and output asymmetries.
CicuitGraphPlot: fix mild error when trying to work with less than 2 neurons.