Added CompoundDataPlug test case demonstrating Box promotion.
Added CompoundDataPlug test case demonstrating Box promotion.

Fixes #1043.

cedriclaunay/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,cedriclaunay/gaffer,goddardl/gaffer,goddardl/gaffer,johnhaddon/gaffer,hradec/gaffer,hradec/gaffer,hradec/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,boberfly/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,chippey/gaffer,chippey/gaffer,boberfly/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,boberfly/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,goddardl/gaffer,andrewkaufman/gaffer,goddardl/gaffer,andrewkaufman/gaffer,chippey/gaffer,hradec/gaffer,GafferHQ/gaffer,chippey/gaffer,hradec/gaffer,cedriclaunay/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer