Added field to choose skeleton line width in the 3d viewer
Classification: add API to export classification ontologies

The new API is available as "classification/<workspace_id>/export_ontology and
returns a JSON representation of all classification ontologies, including all
sub-paths.
Neuron navigator: use new compact-skeleton API

Recent changes of the compact-json API broke the neuron navigator (same
problem as in issue #710). This commit makes the navigator use the
compact-skeleton API and fixes the problem mentioned before.
Neuron navigator: initialize arbor for active skeleton properly
Merge remote-tracking branch 'origin/master' into morphology-pca
Merge remote-tracking branch 'origin/linewidth-setting' into morphology-pca

Conflicts:
	django/applications/catmaid/static/webglapp.js
WebGLApp: fix some of the many whitespace inconsistencies.
Classification Editor: High z-index for imagepreview div

Before this fix, the resize_handle div (z-index=8) from the
window manager would be on top of the image preview in the
Classification tree.
Merge pull request #711 from fzadow/fix-imagepreview-in-classification-editor

Classification Editor: High z-index for imagepreview div
Arbor.js: text editor tab/space setup.
Measurements: use local computations rather than server-side,
and use Gaussian-smoothed arbors rather than 3-point averaged ones
for estimating the smooth cable length.
Add the lower-bound cable length for reference.
Measurements table is now a SkeletonSource.

The major motivation was to manage neuron names with neuronNameService.
Merge branch 'master' of github.com:acardona/CATMAID
compact_skeleton: when a skeleton does not exist, the client
expects a very specific error message to be able to distinguish
this situation from the skeleton being unloadable for other reasons.
measurements table: fix errors in removeSkeletons.
Measurements table: add option button to change sigma for smooth cable measurement.
Measurements: export as CSV
WebGLApplication: createSynapseMap preserves the relation (pre or post).
synapse clustering: cleaner, safer logic that resists 0 being falsy.
synapse clustering: new function segregationIndex

This algorithm by Casey Schneider-Mizell.