Reduce contact patches

Improvement of setup of dualmortarcoupling, such that vertices cannot be contained in more than one contact patch (only membership in several mortar patches is allowd). This is useful in cases when it is difficult to separate the patches of different contacts in advance. If a vertex is contained in more than one patch, it is left in the first (in the order in which the contacts are given) patch and removed from the others. Probably it would be better to consider the gaps/distances of this vertex for the different contacts and choose accordingly the contact patch to which this vertex should belong, but this seemed to be more effort to implement.