Merge branch 'fix-vector-leaf-index' into 'master'

ref:staging/dune-uggrid 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.

See merge request [!141]

  [!141]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/141