[Animator] - animate calls return correct typings

Original interface was incorrect.  In some cases, a D3.Selection is returned.  In some cases, a D3.Transition.Transition is returned.  Ideally an animator should return D3.Selection | D3.Transition.Transition or we have the null animator act as a different type.  For now, just specifying the types to be more accurate

Note: I believe this is API-breaking for typescript consumers since types returned are going to be different.  Unsure in this case.