Added ability to construct LinkedScene from IndexedIO pointer

This functionality really helps me in my big SceneModGraph restructuring project, which I need to do to implement a couple of features people have asked for. I had to add another constructor (the SceneInterfacePtr one) to avoid a silly compiler error in IECorePython::LinkedSceneBinding.cpp (the compiler didn't know weather to use the ConstSceneInterfacePtr or the IndexedIOPtr constructor), which kind of sucks. Could anyone advise about that? Also, is this binary compatible?