This is in line with how the other APIs are named (for example
ObjectDataXD have instance type and instance). This would be very hard
to change later without breaking backwards compatibility, so I'm doing
it now, until the animation APIs get widely used.
This reduces the templated code a bit, as I moved the index assertion to
the *.cpp file. Also the function now returns a reference to avoid
needless copies -- it's a view, but still quite a heavy view.