These are the lines I'm unclear about (not that there are so many to choose from). I expected the 1's in the upper left 3x3 of this matrix to be given by the values in `get_zooms()`, but using those values makes the test script fail (since the zooms are not unity, but `mri_info --vox2ras-tkr` on the test .mgz file has only +/-1's in the upper left 3x3...