Add DAGCircuit method to replace block of nodes with another set of nodes

[ewinston]

Summary

When optimizing circuits one often knows a transformation of a subcircuit into a more desirable form. If the initial subcircuit is listed as a set of nodes of a DAGCircuit this can be replaced by another set of nodes.

Details and comments

This works by using DAGCirucit.replace_block_with_op and then DAGCircuit.substitute_node_with_dag to apply the transformation.

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[mtreinish]

I did push up a PR to rustworkx to add a native method for doing this based on our earlier discussion: Qiskit/rustworkx#823 when that is merged and in a rustworkx release we can update the internals of this DAGCircuit method to use it.

[ewinston]

Thanks @mtreinish! I didn't see that. I can update the PR after it merges.

[coveralls]

Pull Request Test Coverage Report for Build 4545100440

• 25 of 26 (96.15%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 85.358%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/dagcircuit/dagcircuit.py	25	26	96.15%

Totals
Change from base Build 4544978078:	-0.02%
Covered Lines:	67133
Relevant Lines:	78649

---

💛 - Coveralls