Add support for mode_compute=1 under Octave

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.