Do not use 0 if a node or surface is not defined, as it is 0-based and so 0 does…

Do not use 0 if a node or surface is not defined, as it is 0-based and so 0 does have a meaning! E.g. in case of a segment-facet intersection, only 2 nodes are defined for the segment and the third node was set to 0. In meshGRegionBoundaryRecovery we are looping over these 3 nodes to get the node index at the Gmsh side. As a node with index 0 exists, it will be added to the list of mesh vertex errors whereas it has nothing to do with it.