simple dxf reader

代码说明：

The aim of this tip is to deal with DXF files with the most common AutoCAD entities. Line, lwpolyline, arc, circle and spline entities are supported. All the entities in the file are loaded into proper classes when the file is opened. Afterwards, they are drawn on the screen. Highlight and selection of each entity, indication of points, sliding, zoom in, zoom out and pan zoom functionalities are available.