Removed an unwanted print statement in the ImageReaderTest case.
Merge branch 'ImageReaderTestCleanup' into ChannelMaskForNodes
Picking up the downstream changes that allow a ChannelMaskPlugValueWidget to have no channels as a selection (for testing purposes)

Merge branch 'ChannelMaskPlug' into ChannelMaskForNodes
Included the use of the ChannelMaskPlug into the ChannelDataProcessor base class to add a widget that allows only particular channels to be modified. To implement this a new method called channelEnabled has been added to the ImageNode to provide derived classes a way of dictating which channels are enabled. All image nodes have been brought inline to use this convention. A few TODOs in the Grade node that relied on this feature have been completed and removed as a result.
Required update for testing
Merge branch 'ChannelMaskPlug' into ChannelMaskForNodes
Merge pull request #105 from goddardl/ChannelMaskForNodes

ChannelMaskPlug on the ChannelDataProcessor node and channelEnabled() for all ImageNodes.