Update CounterHandler interface

The `CounterHandler` interface could be simplified by a lot just by allowing the user to directly pass the entity instance.

For example, instead of:
``` python
handler.set_counter(15, 'br', 'se', 28)
```
it could become:
``` python
handler.set_counter(15, snapshotEntityOfBibliographicResource28)
```

All required information can be easily extracted from the entity instance!