Fix #690

To define the distance unit, add the "distanceReturn" parameter.

```json
"geo": {
        "latitudeField": "latitude",
        "longitudeField": "longitude",
        "latitude": 51.508515,
        "longitude": -0.12548719999995228,
        "coordUnit": "DEGREES",
        "distanceReturn": "DISTANCE_KM"
    }
```

The possible parameters are:
* DISTANCE_KM for kilometers
* DISTANCE_MILES for miles