Update to: Filter out disconnected path and include a new axon predicate

[napakalas]

1. Added ilxtr.hasAxonLeadingToSensorySubcellularElementIn predicate to identify axons.
2. Handled new terms that belong to the CL namespace.
3. Updated connectivity_models function to return a distinct list.
4. Set connectivity_paths function to return connected paths only as a default setting.
5. Updated npo.py example script to use mapknowledge-0.19.2.
6. Added a function to draw all knowledge, similar to flatmap-server's sckan_connectivity.py.
7. Added a new example script: npo_summary.py.

[dbrnz]

@napakalas it would have been better to have had several PRs for these changes, one for each distinct issues:

1. Add ilxtr.hasAxonLeadingToSensorySubcellularElementIn predicate to identify axons.
2. Handle new terms that belong to the CL namespace.
3. Returning a distinct list from Npo.connectivity_models().
4. Code related to identifying and managing disconnected paths.
5. Updated examples.

So far as this PR goes, I've merged it into a new branch from which I'll cherry-pick things for a new release -- in particular the disconnected path stuff could do with some tweaking.

[napakalas]

Thanks @dbrnz, In the future I will do as you suggested.