Gather demos

[OliviaLynn]

Gathering external demos as described in #48

These are the 3 that run fine after moving/some minor reformatting; the bpz and fsps demos will be following.

Code Quality

• [x] I have read the Contribution Guide
• [x] My code follows the code style of this project
• [x] My code builds (or compiles) cleanly without any errors or warnings
• [x] My code contains relevant comments and necessary documentation

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (bec7a2d) 100.00% compared to head (c15b902) 100.00%. Report is 16 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #73 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 1 1 ========================================= Hits 1 1 ```

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