Paths from unselected nerve centrelines shouldn't show.

[dbrnz]

Highlighting neuron paths by nerve centreline only works as expected if all neuron paths are first hidden. In other words, selecting a nerve centreline needs to hide paths for all unselected centrelines at the same time respecting the selected path types (sensory, sympathetic, etc).

[dbrnz]

As part of this:

1. Removing the present all-on/off centreline control.
2. Add a parameter to the call that highlights paths through a nerve to optionally show the nerve.