Structure_Engine: Implement proper explicit castings for Edges

Would be good to fins a way to through explicit castings, like the ones being used from points to nodes, to be able to reduce the number of create methods for the Panel planar down to just one for the "standard" one, accepting edges and openings.

Currently no way to implement the explicit operator as extensions, but that seem to be in the pipeline for C#8: https://channel9.msdn.com/Blogs/Seth-Juarez/A-Preview-of-C-8-with-Mads-Torgersen

Might have to keep this on hold until then.