Connectivity: make OR default, and rename to "All partners".
AND becomes "Common partners".

Makes sense to have OR as default given the "append" button.
Fix error in Selection Table.
New library file arbor.js to create and manipulate arbors.
An Arbor is defined as a graph:
 * with only directional edges
 * only one root node (one node without parent)
 * no loops.

This data structure allows for a number of high-performance versions
of otherwise slow generic graph algorithms.