Apply a geo=$circle/$center filter. Radius is in meters. Subsequent within() 
		calls replace earlier geo filters on this query.

def within(self, lat, lon, radius):
		'''
		Apply a geo=$circle/$center filter. Radius is in meters. Subsequent within() 
		calls replace earlier geo filters on this query.
		'''
		self._geo = { "$circle": { 
						"$center": [
							factual.common.shared_filter_helpers.GeoScalar(lat), 
							factual.common.shared_filter_helpers.GeoScalar(lon), 
							], 
						"$meters": radius 
						}
					}
		return self