Additional magnetic_field NXlog group in NXsample

In the NXsample base class there is a 'magnetic_field' of type NXlog in addition to the field 'magnetic_field' and the deprecated 'magnetic_field_log' NXlog group:

```
magnetic_field: (optional) NXlog

    magnetic_field.value is a link to e.g. magnetic_field_env.sensor1.value
```

I guess, it is a mistake and should be removed (?).