Crops a 3d image to the bounding box specified.

def crop_3dimage(img, beg_coords, end_coords):
    """Crops a 3d image to the bounding box specified."""

    cropped_img = img[
                  beg_coords[0]:end_coords[0],
                  beg_coords[1]:end_coords[1],
                  beg_coords[2]:end_coords[2]
                  ]

    return cropped_img