Arbor.js: new functions distanceToUpstreamIn and pathToUpstreamIn,
where, starting from a node, accumulate cable length or nodes
until finding a node contained a set of stopper nodes.
Arbor.js: subtreesMeasurements and derivative functions, namely:

subtreesCable
subtreesEndCount
subtreesLoad

All return a map of branch node vs list of numeric measurements, one per
subtree starting at the branch node.

The subtreesLoad is a generic function. When given e.g. a map of nodes vs
counts of input synapses, it will compute then the number of input synapses
at each subtree.
Arbor.js: new function asymmetry and its derivatives, namely:

* asymmetryIndex: a topological measurement of branch asymmetry, defined
by van Pelt et al. 1992 and Uyling and van Pelt, 2002.
* cableAsymmetryIndex
* loadAsymmetryIndex

These functions rely on their equivalent in the subtreesMeasurements.
Note that loadAsymmetryIndex is generic like subtreesLoad.
CircuitGraphPlot: plot new topology and anatomy measurements.
OptionsDialog: fix issue with appendCheckbox, didn't allow to set it as false.
CircuitGraphPlot: PCA using a choice of graph, anatomy, synaptic counts
or synaptic distributions, via the options dialog.
utils.js: fix logic in fetchSkeletons

Was unblocking the UI too early.
CircuitGraphPlot: clear pulldown menus on 'Clear'.