Removing KF after keyFrameCallback()

When a KF is removed after being notified to all the processors by keyFrameCallback(), the PackKeyFrameBuffer::selectPack/selectPackBefore() functions can provide a KF pack which is being removed (it has no StateBlocks and is unlinked from the tree).

Instead of addressing this in the FrameBase::remove() function, I think the easiest way to proceed is to handle this in all PackKeyFrameBuffer functions.