Shader node : Output `IECoreScene::ShaderNetwork`, not `ObjectVector`
Shader node : Output `IECoreScene::ShaderNetwork`, not `ObjectVector`

The new `ShaderNetwork` class provides an API tailored to the representation of shading networks, and will facilitate the addition of features such as a ShaderTweaks node.

andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,appleseedhq/gaffer,hradec/gaffer,hradec/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,boberfly/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,boberfly/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,hradec/gaffer,lucienfostier/gaffer,hradec/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,boberfly/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer