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.