Release 1.3.1

Major improvements and features added! Check the change-log for details:

• 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.