Update addons

They actually operate on acquisitions and not the whole experiment, so there is no need to complicate things and store the addons in experiments. Also the hookup mechanism was kind of awkward:

```python
# acquisitions are defined somewhere before
experiment = Experiment(acquisitions, walker)
addon = Addon(acquisitions)
# This is not necessary anymore
experiment.attach(addon)
# You can still play with them like before
addon.detach()
addon.attach()
```