# Description

In Python, memory management is based on reference counts, where an
object isn't "truly" deleted if it has variables referring to the memory
address. More details:
https://www.pythontutorial.net/advanced-python/python-references/

In Orbit code, we provide "self" to various callbacks, which increments
the reference counts since they "refer" to the object. This means that
when we do:
```python
robot = Articulation(cfg=ANYMAL_C_CFG)
```
The reference count is 3: one for the `robot` variable and two for the
callbacks we launched on separate threads.

Now, if we try to delete the object by doing `del robot`, the reference
count becomes 2. These correspond to the callbacks that were launched.
However, the destructor `__del__()` isn't called yet because it is only
done when all the reference counts are 0.

As a fix, this MR uses a proxy weak reference to "self" when passing it
to underlying callbacks or other classes. This does not increment the
reference count for the object; thereby, when the main object is
deleted, all ts proxy references become invalid as well.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file