Made a basic dense graph
Added type assertion tests for dense graph
Small comment
Made a basic dense graph
Made dense graph have DirectedEdgeList instead of EdgeList
Added tests for dense graph
Changed basic tests to use degree
Changed NewDense to return type, not interface
Added Cost to dense graph
Finished tests for dense graph
Full coverage on densegraph
Resolved conflict I think
Merge branch 'densegraph' of github.com:gonum/graph into densegraph

Conflicts:
	concrete/dense.go
	concrete/densegraph_test.go