Add an option to generate noise after substitution

This is a hack that implements adding noise clauses in 5 lines instead of the original 250+ C program. I think the way it is done works for the kth2dimacs module, which is nothing but a large hack in itself, but it is less clear to me how to do it for the modular parts. Noise is currently defined only for pebbling formulas with substitution, so it can be argued that it should be in the pebbling module, the substitution module or in a module of itself.