Matlab Production Server (MPS) fails when onboarding the dynare package
because of `endif` and `endfunction` which are not available in matlab's
syntax.
Since version 1.6, the optim Forge package has an implementation of fmincon.
Hence we can now use mode_compute=1 under Octave.
This commit also adds tests/optimizers/fs2000_1.mod to the testsuite. It will
be skipped under MATLAB if the optimization toolbox is not there, or under
Octave if optim ≥ 1.6 is not there.
Packages are no longer autoloaded, so testing whether they are "Loaded" does
not work in all cases.
The function now ensures that the package is loaded.