Given a list of state-space models belonging to the ss class, creates a joined system whose output is the sum of the state-space outputs. If wv is not None, this is a list of weights, such that the output is:

y = sum( wv[ii] y_ii )

Ref: equation (4.22) of Benner, P., Gugercin, S. & Willcox, K., 2015. A Survey of Projection-Based Model Reduction Methods for Parametric Dynamical Systems. SIAM Review, 57(4), pp.483–531.

Warning: - system matrices must be numpy arrays - the function does not perform any check!