Overhaul use of __pltopt__.m to correctly check for cellstr, not just cell. * scripts/plot/private/__pltopt__.m: Use modern coding conventions. Eliminate unneeded input checking in subfunctions. Remove unused variables. * scripts/plot/contourf.m, scripts/plot/feather.m, scripts/plot/plotmatrix.m, scripts/plot/private/__bar__.m, scripts/plot/private/__quiver__.m, scripts/plot/private/__scatter__.m, scripts/plot/private/__stem__.m, scripts/plot/stairs.m: Use iscellstr, not iscell, to properly validate inputs to __pltopt__.

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