Ability to not include unused variables

[asmeurer]

By default, if you skip variables, pycosat will include them in solutions, like

>>> list(pycosat.itersolve([[2]]))
[[-1, 2], [1, 2]]
>>> list(pycosat.itersolve([[3]]))
[[-1, -2, 3], [-1, 2, 3], [1, -2, 3], [1, 2, 3]]

This can be annoying, because if your literals end up larger than in order, you have to flatten them out or else you will get more solutions than you need. It would be nice to have an option to not include unused variables. Alternately, pycosat could have a helper function to flatten out something like [[2, 5, 6], [-2, 4, 6]] to [[1, 3, 4], [-1, 2, 4]].

[github-actions]

Hi there, thank you for your contribution!

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Thanks!