ArborParser: return this from all independent parsing methods.
compact-arbor takes a boolean argument for nodes as well.

Now all three json elements are optional: nodes, synapses and tags,
while always returning the empty version of each for ease of coding.
WebGLApplication: coloring synapses by axon-and-dendrite uses now
the number of actual outputs rather than just the number of presynaptic sites.
Indicate colors in the menu item for synapse coloring by axon and dendrite.
Merge branch 'master' of github.com:acardona/CATMAID