OME-TIFF: Temporary pixel size hack/fix

See issue 126 for details. The X and Y dimensions also need to be scaled,
however; they currently represent steps/pixel, so multiply by um/step (1.5
hardcoded; fix this).

This also lets single positions function correctly; row step size might be
0 if the row doesn't represent a stack, so make it minimally 1.

Signed-off-by: Luke Stuyvenberg <stuyvenberg@wisc.edu>