Real cofactor not computed for Davio expansion

[runekrauss]

Within manager operations like restr, the cofactor is computed via cof at the respective level depending on the DD type. While the real cofactor is computed for the Shannon expansion, this is not done for the Davio expansion due to performance reasons. In this context, a "trick" of the form if (vlist[x].t == expansion::pD && a) { return consts[0]; } is applied.

For completeness, a generalized method should nevertheless be provided that computes the real cofactor even in the context of the Davio decompositions.