GroupGraph: new button to split node by axon, backbone dendrite and terminal dendrites.
GroupGraph: first attempt at storing sets of partners at each subgraph node.

For later use in the grow command.
WIP
Small fixes to Arbor.js
Merge branch 'master' into gg-client-splits
GroupGraph: can grow connections of e.g. only the axon or only the dendrite,
or arbitrarily of any subnode of an exploded arbor, be it by axon/dendrite
or synapse clustering or other means.
Merge branch 'master' into gg-client-splits
GroupGraph: when removing a node the subgraphs for it must be deleted.
GroupGraph: find paths supporting subgraph nodes as sources or targets.
Merge branch 'master' of github.com:acardona/CATMAID