Addressing #180 (closed): When a KF is removed after being notified to all the processors by keyFrameCallback(), the PackKeyFrameBuffer::selectPack...() functions can provide a KF pack which is being removed (it has no StateBlocks and is unlinked from the tree).
Since the decentralized structure of the KF callbacks, handling this within FrameBase::remove() is quite complicated.
I rather add a check in the selectPack() and selectPackBefore() functions. They are called by the processors to get the notified KF since the last process().
Appliying #163 (closed): Changed the implementation and the name of the function selectPackBefore() (see issue for more info).
All tests OK
Merging to devel