Arbor.js: resampleSlabs can now cope with very jittery arbors,
and has a simpler _gaussianWeights function that doesn't require
concatenating strings to create object keys.
Arbor.js: fix 2 errors in _resampleSlab

1. could have added last node twice.
2. was not cloning reused Vector3 instances.
Arbor.js: add comment about future work.
Fix error in measurement table: counts of branch and ends was swapped.
WebGLApp: introducing options to resample the skeleton.

Root, branch and end nodes are not altered. Slabs are resampled so that
nodes have a maximum, constant interdistance of "resampling_delta", in nm.

Reduces e.g. a neuron with 1000 nodes to about 50 nodes when resampling
to a delta of 5000 nm.

At extremely large deltas, the arbor is reduced to root, branch and end
nodes, linked by straight lines between them.