WIP: Connectivity diagram

Open username-removed-284116 requested to merge friisj/ase:connectivity_diagram into master

ENH connectivity plotting

This adds a module utils/connectivities.py for easy listing and plotting of connectivities. It also adds a tools menuitem in the gui for plotting of connectivity diagrams.

The connectivities module works fine, but would benefit from some cleanup. Especially would it be nice to use covertree.py for finding nearest neighbours, which make the search for nearest neighbours much faster for large systems.