conditional_forecast should save its output in oo_

Currently, conditional_forecast stores its output in a file. This is inconsistent with the rest of Dynare commands, and renders the post-processing of this data less robust, because we have to reload that file from the hard drive (without the guarantee that it comes from the same Dynare session).

The output should be saved in a field of oo_ (without changing the structure).

plot_ic_forecast needs to be adapted. shock_decomposition as well, once #1657 is done.

The various fields should also be documented.