return lat,lon for nodes in PlaceLookup

Fix for https://github.com/twain47/Nominatim/issues/190

PlaceLookup skips some geocoding logic because we already know exactly the place the user is looking for. For nodes we don't store the centroid in the database (placex table) because it would be same as the geometry anway.
Added logic to use the geometry if the centroid is not available.