Arbor.js: fix indentation in a couple of functions.
Arbor.js: Gaussian convolution of slabs to compute smooth cable length
and also to smooth the position of skeleton nodes.
WebGLApplication: add option to smooth skeletons with a Gaussian convolution.

It is surprisingly fast and looks great, removing all jitter.

Could be made even faster by caching distance measurements.
WebGLApplication: refresh skeletons when toggling Gaussian smoothing.