Add property exposing leaves of DAG in AdjacencyList

[jacanchaplais]

Description

When AdjacencyList represents a whole event, AdjacencyList.leaves should give the same result as Graphicle.final. However, when the AdjacencyList is subscripted, or represents other data, AdjacencyList.leaves will enable dynamic access to the dangling edges.

Implementation details

These may be found by considering nodes with an out-degree of zero. If each row is reduced to its sum, this will give the out-degrees of each interaction vertex. The vertex ids where the out-degree is zero may then be found, and a mask over the edges may be formed by seeing which edges "out" vertex id matches the zero out-degree vertex ids.

Notes

Also add some more documentation to the AdjacencyList.