Implement Graph.VertexCover()

[colm-mchugh]

Currently failing test: graph.VertexCoverTest()

Requires a representation for graph edges that enables constant time access to any edge of the graph,
and O(E) removal of all incident edges on any given vertex (E = #edges into & out of the vertex).

[colm-mchugh]

Initial implementation added in this commit.

[colm-mchugh]

Test resources:

• small
• big

[colm-mchugh]

Started adding small tests.
TODO: fix vertex cover on directed graphs
TODO: use a set instead of heap for storing edges (O(E) instead of O(E*log(E))

[colm-mchugh]

Running vertex cover on graph with 500 vertices, 99K edges:
heap implementation:
Undirected: size=300 millis=443
Directed: size=478 millis=143

set implementation:
Undirected: size=452 millis=225
Directed: size=456 millis=85

TODO: add vertex cover verification