Decide on whether evalute_smoother (and others) should set M_.params

This picks up the discussion in https://github.com/DynareTeam/dynare/pull/1372#issuecomment-271336355

I agree with @MichelJuillard that changing M_.params in various functions is a bad idea as it makes it hard to trace what was going on in the mod-file. At the same time, I agree with @rattoma that keeping M_.params unchanged when calling evaluate_smoother would imply a break with previous versions and therefore mean a loss in backward compatibility. I could live with that, but usually @MichelJuillard prefers to be cautious in the regard. If we want to preserve backward compatibility, we need to make sure that shock_decomposition returns M_ from evaluate_smoother to the base workspace, which was broken by https://github.com/DynareTeam/dynare/commit/2f717b5adc5a87f663c5c080f2963d1f65d1933e