The SketchPlane expects the first element to be a planar face/edge to
define a work plane for any following draft wire/circle/arc elements.
The 2D constrainted elements can either be inside the defining
SketchPlane constraint, or other constrains following the SketchPlane
in the constraint group.
Add a new SketchPlane to define a new work plane for any following
draft elements. Add an empty SketchPlane to undefine the work plane,
effectively making any following draft elements free in 3D space.
When added to an assembly, a normal part object has six degrees of
freedom, three positional and three rotational. A draft circle has one
additional dof, its radius. And a draft arc has three additional dof,
radius, first angle and last angle.
