Change in plasticity model

The return algorithm of the plasticity model has changed in version 3.3.0 (06/09/14). The BETA parameter of the NonLinCementitious2 model now has a different impact on the final results. Specifically, if a standard Poisson's ratio of 0.2 is used, the BETA parameter of the new solver is equal to half of the BETA parameter of the old solver. Hence, all old models with nonzero BETA should be updated.