Log Message:
Updated cad/py-gds from 1.2.1 to 1.3.1

### Version 1.3.1 (Jun 29, 2018)
* `PolygonSet` becomes the base class for all polygons, in particular `Polygon` \ 
and `Rectangle`.
* Added `Cell.remove_polygons` and `Cell.remove_labels` functions to allow \ 
filtering a cell contents based, for example, on each element's layer.
* Added `PolygonSet.scale` utility method.
* Added `PolygonSet.get_bounding_box` utility method.
* Added argument `timestamp` to `Cell.to_gds`, `GdsLibrary.write_gds` and \ 
`GdsWriter`.
* Added `unit` and `precision` arguments to `GdsLibrary` initialization and \ 
removed from its `write_gds` method.
* Changed the meaning of argument `unit` in `GdsLibrary.read_gds`.
* Improved `slice` to avoid errors when slicing in multiple positions at once.
* Improved `PolygonSet.fracture` to reduce number of function calls.
* Removed incorrect absolute flags for magnification and rotation in \ 
`CellReference` and `CellArray`.
* Minor bug fixes.
* Documentation fixes.
* Removed deprecated classes and functions.