Added a CoordinateSystem node to GafferScene.
Implemented drawing of coordinate systems in the viewport.

Unfortunately this continues the abuse of the SceneProcedural, but that is inevitable until we have a more sophisticated viewer.
Added coordinate system output functions to RendererAlgo.h.
Added coordinate system support to ExecutableRender.
Added coordinate system support to InteractiveRender.

This is just preliminary support and doesn't have unit tests yet. Currently it only works if the Renderer::coordinateSystem() call in IECoreRI::Renderer is modified to use RiCoordinateSystem() instead of RiScopedCoordinateSystem().
Added unit tests for moving coordinate systems during IPR.

This requires 3delight 11.0.96 to pass successfully.
Merge pull request #946 from johnhaddon/coordinateSystems

Coordinate systems