Some issues with ZDW finding statistic

[chrisbrahms]

As per #211 (comment)

If I remember correctly, the issues were:

handling of missing in the HDF5 backend (when no ZDW found)
making the lower bound lb in the ZDW search depedn on the grid.

[jtravs]

I think I have a solution to the latter case (in which depending on the grid is annoying I think).