Geohash filter enhancement

The `geohash_cell` filter now adapts the format of other geo-filters. The oject fieldnames match the fieldnames document names automatically. This invalidates the `field` field in previeous versions. The value these fields value is a `geo_point` value (all formats supported) which is internally translated to a geohash. Since those points alway have a maximum precision (level 12) a `precision` definition has been included. This precision can either be defined as *length* of the geohash-string or as *distance*. It's assumed the a distance without any unit is a geohash-length.

```
GET 'http://127.0.0.1:9200/locations/_search?pretty=true' -d '{
    "query": {
        "match_all":{}
    },
    "filter": {
        "geohash_cell": {
			"pin": {
				"lat": 13.4080,
				"lon": 52.5186
			},
            "precision": 3,
            "neighbors": true
        }
    }
}'
```
Closes #3229