ENH: handling of open edges distanceSurface at 0 (issue #948)

Some special adjustments are undertaken for distance = 0.

- With the isoSurfaceCell algorithm is used, additional checks for open
  surfaces edges are used to limit the extend of resulting distance
  surface. The resulting surface elements will not, however, contain
  partial cell coverage.

- Always treated as signed (ignoring the input value), since it is
  nearly impossible to generate any surface otherwise.