Add DAGCircuit method benchmarks.

[kdk]

Summary

Adds two benchmarks for DAGCircuit.apply_operation_back and DAGCircuit.substitute_node_with_dag.

Details and comments

WIP - Because substitute_node_with_dag will modify the dag built in setup, subsequent iterations of the benchmark from asv would fail to find the target node. I attempted to work around this by creating N dags in setup, but this is slow. Also, this requires limiting ASV to a fixed number of iterations for a given benchmark, and I wasn't able to get this to work locally.

[HuangJunye]

@kdk Are you still working on this PR?

[Eric-Arellano]

Closing as stale after talking to Kevin on DM.