Activity should be able to be called outside of subscriber

### Description

An Activity should be able to handle being called without being a subscriber callback.
That is, you should be able to call an activity.

Example:
```
class MyActivity(Activity):
pass
activity = MyActivity(...)
activity()
# Stuff happens
```

### Motivation
Sometimes an activity can be triggered without listening on events, for example when creating a tool that sends activity triggered,started,finished.

### Exemplification

Activities should be allowed to be used as activities started and not just activities triggered.

### Benefits

Ease of use.

### Possible Drawbacks

Maybe pinning too much functionality to the activity object.

Beginnen Sie damit, die Activity-Implementierung und den Pfad des subscriber callback in der Python library zu finden. Ermitteln Sie, wie direkte Aufrufe zu den Ereignissen triggered, started und finished in Beziehung stehen sollten, und überprüfen Sie anschließend, dass das bestehende Verhalten der subscriber unverändert bleibt und dass der direkte Aufruf durch Tests abgedeckt ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.