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.
Add unit test for order=3

The unit test uses the policy rules of Fernandez-Villaverde et al.
(2011)'s "Risk matter" derived from Mathematica to compare them to the
Dynare matrices. If the policy functions differ by more than 1e-9, an
error is produced.
Remove pruning_forecast_approximation introduced in cdb7b018798da5e137414e611ea68cf097a62036
Merge pull request #376 from JohannesPfeifer/master

Add Andreasen et al pruning at order=3, add unit test for third order perturbation