Add seeded topological sort to DiGraph

This adds a method to DiGraph called "seededLinearize". This
generalizes the original topological sort ("linearize") to be
parametric in an initial set of vertices. This enables the user to
massage the DFS to produce a better topological sort if they have
information about how the DFS should proceed.

The initial set of vertices is expected to be made ordered via a
LinkedHashSet.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>