Added and tested unmark_variable() method.

examples/scripts/sellar_problem/(X)DSM/FPG_IDF.pdf

examples/scripts/sellar_problem/(X)DSM/FPG_MDF-GS.pdf

examples/scripts/sellar_problem/(X)DSM/FPG_MDF-J.pdf

...es/scripts/sellar_problem/(X)DSM/FPG_converged-DOE-GS.pdf

...les/scripts/sellar_problem/(X)DSM/FPG_converged-DOE-J.pdf

...es/scripts/sellar_problem/(X)DSM/FPG_converged-MDA-GS.pdf

...les/scripts/sellar_problem/(X)DSM/FPG_converged-MDA-J.pdf

.../scripts/sellar_problem/(X)DSM/FPG_unconverged-DOE-GS.pdf

...s/scripts/sellar_problem/(X)DSM/FPG_unconverged-DOE-J.pdf

.../scripts/sellar_problem/(X)DSM/FPG_unconverged-MDA-GS.pdf

...s/scripts/sellar_problem/(X)DSM/FPG_unconverged-MDA-J.pdf

.../scripts/sellar_problem/(X)DSM/FPG_unconverged-OPT-GS.pdf

...s/scripts/sellar_problem/(X)DSM/FPG_unconverged-OPT-J.pdf

examples/scripts/sellar_problem/(X)DSM/MDAO_IDF.pdf

examples/scripts/sellar_problem/(X)DSM/MDAO_MDF-GS.pdf

examples/scripts/sellar_problem/(X)DSM/MDAO_MDF-J.pdf

...s/scripts/sellar_problem/(X)DSM/MDAO_converged-DOE-GS.pdf

...s/scripts/sellar_problem/(X)DSM/MDAO_converged-MDA-GS.pdf

...es/scripts/sellar_problem/(X)DSM/MDAO_converged-MDA-J.pdf

.../scripts/sellar_problem/(X)DSM/MDAO_unconverged-DOE-J.pdf