Change format of returned regions for easy parsing

New format example:
```json
"regions": [
            {
               "lat": -22.5,
               "lon": 22.5,
               "count": "70"
            },
]
```
Add support for additional grouping

Added parameter "additional_grouping" for geohashcell facet. When this
parameter is provided, the region counts are additionaly broken down
according to terms in this field.