Implement Clone and Debug for MinCostFlowGraph and _Edge

[NotLeonian]

Implement the Clone and Debug traits for MinCostFlowGraph.
This requires implementing Clone and Debug for _Edge as well.

To implement this change, we first need to choose between setting the trait bound for T as Clone, Debug or as Integral.
I think the former seems better.

For the Clone trait, it should be sufficient to specify the trait bound and write the impl.
For the Debug trait, discussion is needed regarding the output format.
I think using the debug_struct function for both MinCostFlowGraph and _Edge should be enough.

[NotLeonian]

To maintain consistency with other files, if both this issue and #153 are implemented, I think that one of them should include the necessary use statement for std::fmt.