bug fix: when resampling, must scale displacements

[GFleishman]

The evaluation script takes a --sampling_factor argument which is passed to zoom to rescale the inputs. If the displacement vector field is resampled, it must also be scaled by the sampling_factor. Consider a displacement vector of [1, 1, 1] before resampling. This means move one voxel along each axis. Now consider resampling the data with a sampling_factor of 0.5. All voxels are now twice as big. Using the original value of [1, 1, 1] is now double the amount of displacement it was before resampling. So, what you really want is [0.5, 0.5, 0.5], which is: disp_field *= SAMPLING_FACTOR which I have added to line 73 of evaluation.py.

[GFleishman]

Incidentally, if you maintain transforms in physical units and always give the physical voxel spacing of image and transform grids to the resample function these issues are less finicky. I see the appeal of using voxel units (because map_coordinates and it seems simpler) but in real applications all image measurements (registrations, segmentations, etc.) should always be in physical units.

[mansouralawi]

Thanks @GFleishman for submitting this fix! as mentioned the issue was due missed rescaling step after the downsampling and your solution would resolve it. Appreciate your help!