Routines to find the boundary of a set of nodes.
An edge boundary is a set of edges, each of which has exactly one endpoint in a given set of nodes (or, in the case of directed graphs, the set of edges whose source node is in the set).
A node boundary of a set S of nodes is the set of (out-)neighbors of nodes in S that are outside S.
||Returns the edge boundary of
||Returns the node boundary of