DMET Demo (#1330)

**Title:**
Density Matrix Embedding Theory (DMET) Demo
**Summary:**
This demo outlines the basics of density matrix embedding theory, and
provides instructions on how to execute the method.
**Relevant references:**

**Possible Drawbacks:**

**Related GitHub Issues:**

----
If you are writing a demonstration, please answer these questions to
facilitate the marketing process.

* GOALS — Why are we working on this now?

*Eg. Promote a new PL feature or show a PL implementation of a recent
paper.*


* AUDIENCE — Who is this for?

*Eg. Chemistry researchers, PL educators, beginners in quantum
computing.*


* KEYWORDS — What words should be included in the marketing post?


* Which of the following types of documentation is most similar to your
file?
(more details
[here](https://www.notion.so/xanaduai/Different-kinds-of-documentation-69200645fe59442991c71f9e7d8a77f8))
    
- [x] Tutorial
- [ ] Demo
- [ ] How-to

---------

Co-authored-by: soranjh <soran.jahangiri@gmail.com>
Co-authored-by: Soran Jahangiri <40344468+soranjh@users.noreply.github.com>
Co-authored-by: agran2018 <45397799+agran2018@users.noreply.github.com>
Co-authored-by: ixfoduap <40441298+ixfoduap@users.noreply.github.com>
Co-authored-by: Josh Izaac <josh146@gmail.com>
Co-authored-by: Daniela Angulo <42325731+daniela-angulo@users.noreply.github.com>

Author: 6 people