search: remove weight graph.CostFunc parameter from path search functions

[kortschak]

This can be done by, e.g.

type costModGraph struct {
    graph.Graph
    cost graph.CostFunc
}

func (g costModGraph) Cost(e graph.Edge) float64 {
    return g.cost(e)
}

and wrapping the graph.

This simplifies the API and the internal code and reduces the number of times we do search.PathFinder(g, nil), which seems pretty common (I see no case in our code where the cost function is provided as a graph.CostFunc rather than as intrinsic to the graph).