Generalize HEFT & CPOP

[jaredraycoleman]

Generalize HEFT and CPOP to accept arbitrary cost functions:

exec_cost(task: nx.NodeView, node: nx.NodeView) -> float: returns the cost to execute task on node.
comm_cost(dependency: nx.EdgeView, edge: nx.EdgeView) -> float: returns cost to communicate dependency's data over network edge.
By default, HEFT tries to use weight features and does task_graph_weight / network_weight for both exec and comm costs.