Add TagFilter node

[bentoogood]

The cortex scene cache format supports metadata tags attached to items in the cache.
These could be used to filter the paths in a scene, return items that have the specified tag/s (or those that don't have said tags)

[bentoogood]

I believe this also requires the SceneReader node to be read the tags from the scenecache into the gaffer scene.

[johnhaddon]

I think tags will probably just be represented in gaffer as attributes, so this could perhaps just be an AttributeFilter. Or perhaps derive from one for the functionality but provide a more specific UI.

This needs quite a lot of thought in order to keep bounds propagation working properly.

[johnhaddon]

Needs #494.

[bentoogood]

Depends on #145 (most likely a subclassed AttributeFilter with a special interface for working with tags)

[andrewkaufman]

@johnhaddon is this ticket still relevant?

[johnhaddon]

I don't think so no. I think something equivalent will get done for #92, but it won't be a TagFilter node - it'll either be a SetFilter or an additional mode to the existing PathFilter. I'll close this one as invalid...