Add a playground

📜 To add experimental scripts or usage examples, without testing everything : simply add your own playground in qmat.playgrounds.

create a folder with a short & representative name, e.g yoodlidoo (can also be your name for a personal playground),
put your script(s) in it, and document them as much as necessary so anyone else can understand and use your code,
create a __init__.py file in your playground folder with a short summary of your scripts in its docstring, e.g
```
"""
- :class:`script1` : trying some stuff.
- :class:`script2` : yet another idea.
"""
```

add the item line corresponding to your playground in qmat.playgrounds.__init__.py, e.g

"""
...

Current playgrounds
-------------------

- ...
- :class:`yoodlidoo` : some ideas to do stuff
"""

open a pull request against the main branch of qmat.

💡 If you don’t want your playground to be integrated into the main branch ofqmat (no proper documentation, code always evolving, …), you can still add a soft link to a playground in your fork by modifying qmat.playgrounds.__init__.py :
"""
...

Current playgrounds
-------------------

- ...
- `{name} <https://github.com/{userName}/qmat/tree/{branch}/qmat/playgrounds/{name}>`_ : some ideas ...
"""
where name is your playground name, userName your GitHub username and branch the branch name on your fork you are working on (do not use the main branch of your fork ⚠️)