AdditiveGroup instances for Angle types
Remove Angle constraint on Spherical type

avoid Datatype Context
New Camera type & CameraLens class; perspective & orthographic instances
Add types PointLight, ParallelLight

PointLight represents a spherically symmetric light source
ParallelLight represents parallel light rays covering the entire scene
Export Camera and Light modules
Directions do not form an AdditiveGroup
Merge pull request #114 from diagrams/threeD-render

Three d render
removed shaftWidth, now use shaftStyle = lw ..., and added connectOutside
Use point envelope (@ origin) for text objects instead of empty.

Fixes #115.
Fixes #116.

NOTE: make sure we update documentation to match/explain this more carefully.
Merge branch 'master' into arrows
D.TwoD.Arrowheads: some documentation + warning cleanup