GafferScene.Parent node

This allows the parenting of one hierarchy under a specific location in another hierarchy. This is quite an improvement since to date our only means of combining two hierarchies was to group them together under a new root. Incoming child items are renamed to prevent clashes with existing items by appending numeric suffixes where necessary.

The Parent node is implemented on top of a beefed up BranchCreator, which has incidentally improved the usefulness and robustness of the existing Instancer and Seeds nodes. There is some overlap with code in the Group, but I've made some notes in a \todo item with some ideas as to how this could possibly all be unified under a new HierarchyProcessor base class. I think the intermediate mapping implemented here also brings us one step closer to allowing multiple parents in the BranchCreator node - which if implemented would make all these nodes much more powerful.