Fix EigerDecoder status

`name` is a reserved key in the parameter tree, so parameters cannot be called `name`. Rename to `class` [here](https://github.com/DiamondLightSource/eiger-detector/blob/master/cpp/data/frameReceiver/src/EigerFrameDecoder.cpp#L545).

This is what causes these errors in fastcs-eiger when it ignores them.

```
WARNING:root:Removing parameters with invalid names: ['0/status/decoder/name']
WARNING:root:Removing parameters with invalid names: ['1/status/decoder/name']
WARNING:root:Removing parameters with invalid names: ['2/status/decoder/name']
WARNING:root:Removing parameters with invalid names: ['3/status/decoder/name']
```

Could also add some other relevant status here now that it is exposed in fastcs

- detector_model_
- buffer_size
- frames_allocated_
- current_frame_number_
- dropping_frame_data