More robust sampling

[mcquin]

Sample the corner with the lowest standard deviation. Pad the image with random noise generated from the corner's mean and standard deviation.

Additionally, reorganize preprocessing operations to reduce noise from omitted pixels:

1. Crop the image such that the dimensions are no more than the desired size.
2. Clip the pixels in the 0.5th and 99.5th percentile, reducing noise in the cropped image.
3. Pad the image with random noise sampled from the mean and standard deviation of the corner with the lowest standard deviation.
4. Stretch the image from (image.min, image.max) to (0, 256), and save as uint8.