private setProblem() ensuring viral call
Now, in all wolf nodes the function setProblem(ProblemPtr) is public. Inside this function, eventually things (state blocks, factors) are registered to the problem.
Once it is called in one node, it is viral, it is called also for the children nodes.
We should enforce that only the viral call is used started when a node is linked to something (a frame, the trajectory..) that is connected to the problem. So, forbidding the user to manually call setProblem() to a node which could be dangerous if the node is not indeed linked to the tree.
Related with #228 (closed): Factors in a non-key frame not allowed or not registered