http://www.elasticsearch.org/guide/reference/query-dsl/geo-distance-filter.html
        Filters documents that include only hits that exists within a specific distance from a geo point.
        field - Field name
        center - Center point (Geo point)
        distance - String for the distance
        distance_type - (arc | plane) How to compute the distance. Can either be arc (better precision) or plane (faster). Defaults to arc
        > bounds = ElasticFilter().geo_distance('pin.location', [40.73, -74.1], '300km')

def geo_distance(cls, field, center, distance, distance_type=None):
        '''
        http://www.elasticsearch.org/guide/reference/query-dsl/geo-distance-filter.html
        Filters documents that include only hits that exists within a specific distance from a geo point.
        field - Field name
        center - Center point (Geo point)
        distance - String for the distance
        distance_type - (arc | plane) How to compute the distance. Can either be arc (better precision) or plane (faster). Defaults to arc
        > bounds = ElasticFilter().geo_distance('pin.location', [40.73, -74.1], '300km')
        '''

        instance = cls(geo_distance={'distance': distance, field: center})
        if distance_type is not None:
            instance['geo_distance']['distance_type'] = distance_type
        return instance