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.