This commit adds a new class template argument,
bool PermitSelfConnections
with a default value of 'false', that enables client code to
configure whether or not function
addConnection(v1, v2)
creates a connection from v1 to v2 if v1 == v2. The default state
preserves the current behaviour which ignores such connections.
Permitting self connections makes the class slightly more general to
uses cases outside inter-region flow accumulation.
8a4735efcb