Bug in guessing of bounding box

[philippotto]

E.g., when only having the cube at x=40, y=18, z=1 (mag 0) for ds 2017-05-31_mSEM_scMS109_bk_100um_v01, the initial position for view is outside of the data, even though though the cube is dense.

One potential problem is that the bounding box is calculated by looking at cubes in higher magnification which might be sparse, but I think this is not the problem in the above described case.

[philippotto]

Feel free to re-assign, @fm3.