time lagged models

[levisc8]

Still not implemented. I suspect the underlying code/notation will/should look like the age X size example in #5. This interface will also require adding a pronoun to access the iterator in .iterate_model.

n_z_t_1 = F %*% n_z_[t-1] + P %*% n_z_t

Think carefully about how to have the user specify initial conditions here - needs to remain intuitive and really shouldn't require much additional code to specify the model.