Procedural Edge to Spline implementation

[Rick Barrett]

See https://bitbucket.org/cineversity/cv-selection-spline/src/960f8219e5746960acc07cea640f56087e53d9bc/cv-selection-spline.pyp?at=master&fileviewer=file-view-default

Maybe include a few default selections, like "Border Edges" and based on Phong Angle.