Expose Plane property on BND_Arc

[coditect]

This pull request exposes the Plane and Normal properties of Arc objects, as well as a setter for the Arc property of ArcCurve.

[sbaer]

I would remove Arc.Normal and ArcCurve.Arc (setter). I'm trying to keep this library as close to what we have in RhinoCommon as possible.
https://developer.rhino3d.com/api/RhinoCommon/html/Properties_T_Rhino_Geometry_Arc.htm
https://developer.rhino3d.com/api/RhinoCommon/html/P_Rhino_Geometry_ArcCurve_Arc.htm

[coditect]

I'll remove them for the sake of consistency, but I think they would be worthwhile additions to RhinoCommon because:

• Circle has a Normal property, so why not Arc (which is a subclass of Circle in openNURBS)?
• LineCurve.Line has both a getter and setter, so why not ArcCurve.Arc?

[sbaer]

Documentation for this project is created by scraping the RhinoCommon API documentation. If we add properties, we need to start with RhinoCommon and then add them to this project. It really is just a matter of consistency.