Factors in a non-key frame or null frame

In the context of a factor:

• Linking means adding shared_from_this to the parent's list of children (Feature) and keep its pointer. Also, for the other "constrained" wolf nodes (Landmark, Frame,...).
• Registering means notifying Problem about the existence of this, to update the solver.

Then, in two cases the factor shouldn't be registered:

1. It is not indirectly linked to Problem: this->feature->capture->frame->trajectory->problem any of these nodes is nullptr.
2. The frame is not a key-frame, this frame is not estimated (not in the solver).

For solving 1, the related issue #233 (closed) would be enough.
For solving 2, some work has to be done in:

• FactorBase::setProblem() checking if the frame is key before calling getProblem()->notifyFactor(shared_from_this(),ADD).
• FrameBase::setKey()/setAux() notifying the addition of the factors of this frame. (Analogously, FrameBase::setNonEstimated() should notify the removal of the factors).