ENH: Add speed-up rotator block for rotating several nodes similarly

The transformation block RotateMultiple makes it possible to perform the
same rotation for multiple nodes. Usually one performs the rotation for
one node and the inverse rotation for another node, X->R*X and
C->inv(R)*C. However, using RotateMultiple, it is possible to rotate,
for instance, as X->R*X, C->inv(R)*C, B->inv(R)*B, A->inv(A)*A by using
RotateMultiple(C, B, A).
ENH: Add zero lower bound term for deterministic nodes

It is more consistent if all nodes have lower bound term method, so
deterministic nodes have it now too, it just returns zero because a
deterministic function is a delta distribution, that is, all mass at a
single point.
DEMO: Refactor drifting LSSM demos

The model construction is now moved to a separate module model_lssm.
Thus, it is easier to re-use the model and to see what is model specific
and what is problem/application specific.