Density matrix backend

[shinich1]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run pytest)
• format added code and tests by black -l 120 <filename>
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.

Then, please fill in below:

Context (if applicable): na

Description of the change: Add density matrix simulator backend and interface for custom noise model.

Related issue: closes #31

also see that checks (github actions) pass. If lint check keeps failing, try installing black==22.8.0 as behavior seems to vary across versions.