Deltavision: conditionally ignore stage positions values of 0

If only two unique positions are present in X and Y, and one of them is
0 in both cases, then we ignore the 0 positions (with the result being
that there is only one tile).  See gh-1320.
Merge pull request #1324 from melissalinkert/dv-stages

Deltavision: conditionally ignore stage position values of 0