Log Message:
(cad/py-gds) Updated 1.5.2 to 1.6

### Version 1.6 (Aug 12, 2020)
* Added support for element properties.
* Added transformation support to `Cell.copy`.
* Layer/datatype filtering in `get_polygons` for `Cell`, `CellReference` and \ 
`CellArray`.
* Layer/datatype filtering in `LayoutViewer`.
* Removed global cache `_bounding_boxes`.  Only cells cache their bounding boxes.
* Bug fixes (thanks Daniel Hwang for the contributions).
* Bug fix in `Cell.copy` where the whole dependency tree would be copied on a \ 
deep copy creation.