Implement Andreasen et al. 2013 pruning at order
Implements and documents the Andreasen et al. 2013 pruning at order=3 and sets it as the default at this order. Michel's pruning based on the approximation of the forecast function has been assigned the option pruning_forecast_approximation. The preprocessor-interface still needs to be added for this option. Moreover, more documentation/a reference what this option does is needed. At a later point, we might change the default to Michel's approach.