Removed stubs for Grade Op.

Now there's a proper Grade node there's no reason to use the Op, and it just makes the node menu search results annoying to have them there.
Fixed up Grade's includes.

They were out of order, and including a bunch of unnecessary things.
Removed pointless Context operations from Grade::hashChannelData().

It was getting the channel name from the current context, and then making a new context only to put exactly the same channel name into it.
Fixed Grade channel-independence hash bugs.

All three components of the colour plugs that control the grade were being hashed in for each channel, when in reality only a single component affects each channel. Fixing this should result in faster interaction and smaller caches when the user is tweaking a single channel at a time.

Fixes #338.
Made Grade::channelEnabled() disabled for default plug values.

This means that it is a pass-through for the most common case in the image viewer, yielding improved performance. Also now accessing only the components of the input plugs that are needed for each channel, rather than getting the colour and then extracting the component - this also has a small performance improvement.
Fixed GafferImageTest.ConstantTest.testTileHashes().

It was comparing the hashes of the data returned from channelData(), rather than the results of channelDataHash(). The former would be very hard to get wrong, but the latter is easily got wrong.