Fixes for new SceneNode test cases

Previously, SceneNode::compute would be special cased for the scene root, but these special cases would not be reflected in SceneNode::hash, leading to caching problems. The solution is to hash the default plug values for the special cased plugs at the root. The hash for a transform at the root is maybe a bit odd at the moment, but I'll put it in as a placeholder.
modified SceneTestCase.assertSceneHashesNotEqual to ignore cases where hashes are automatically equal
fixed a few test cases by removing AlembicSource::hash and replacing it with hashBound, hashTransform and hashObject