new API to make is possible to adjust default scale factor in each TwistyObjectNode.(and use it in Tutorials, where the object needs to be smaller)
Major change: separate the notion of a TwistyObject and its Node. Now,
1) the Node stays when we change objects (this makes transitions faster)2) it's possible to just create an Object without adding it to the Screen (now app needs to explicitly create the Node and add it to its Screen itself)