CircuitGraphPlot: set default PCA parameters to the subset of strongest ones,
which are a mix of absolute and relative measurements.

The strongest, in order from most to least:
 * segregation index
 * cable hillock
 * output asymmetry index
 * cable length
 * normalized histogram of output asymmetry
 * normalized histogram of input asymmetry
 * input asymmetry index
CircuitGraphPlot: made it robust to no neurons being connected to each other.

The pulldown menus display then only anatomical/synapse measurements.
CircuitGraphPlot: isolate memory-consuming operations to help GC
and also to reduce the clutter of variables a lot.
CircuitGraphPlot: when a skeleton fails to load, pretend all its values are zero.