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

* Added the ChannelMaskPlug to the ChannelDataProcessor. This adds a nice UI element that allows the user to select which of the input channels should be affected by the node. 
* To implement the enabling of channels a new method called channelEnabled has been added to the ImageNode base class. This method is called when processing the hash or compute of the channelDataPlug(). 
* All image nodes have been brought in line to use the channelEnabled method where necessary.
* All image node's enabled and channelEnabled methods are now protected.
* The TODOs that were in the Grade node that relied on the above features have now been removed.