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This fixes crashes under Octave (while MATLAB seems to accept array-indexing
with cell arrays).

Thanks to Reuben for reporting.

fs2000_uncorr_ME_missing was failing on some servers due to a numerical
precision issue. It appears that the problem drops off if the smoothed
variables are computed on another point (given by another optimization
algorithm).

See https://forum.dynare.org/t/simulated-kurtosis-reported-incorrectly/12534

Adapt the k-order DLL as a consequence.

Updated preprocessor submodule.

This optimizer seems more robust accross different hardware (with other
optimizers, the algorithm does not converge to a local maximum, e.g. on the
runner "lupi").

- update preprocessor submodule
- add some tests

This reverts commit f1dbd8be.

The files were already added in 8400a952.

The "extends" keyword is not yet in Gitlab 11.2 (it is said to be implemented
in 11.3…).