Rentroduce Vector::skip as Vector::leafIndex

The 'skip' data member of the Vector class used to be abused for the leaf index of the vector, if that vector corresponds to an element facet in 3d. By accident, that 'skip' field got removed in a recent commit. Let's put it back in, but rename it to 'leafIndex' now. All it is used for nowadays is that leaf index.

merged

mentioned in commit 172f6988

mentioned in issue #47 (closed)