Projection length in months
proj_len()returns how many months the projection for each model point should be carried out for all the model point. Defined as:
np.maximum(12 * policy_term() - duration_m(0) + 1, 0)
Since this model carries out projections for all the model points simultaneously, the projections are actually carried out from 0 to
max_proj_lenfor all the model points.