Add subtract background preprocessing

[danifranco]

Something like:

from skimage import io
from skimage.restoration import rolling_ball
from skimage.transform import rescale
from skimage.util import img_as_ubyte, img_as_float
from scipy.ndimage import uniform_filter

def subtract_background(img,radius=31,scale=8):
    img = uniform_filter(img, size=3)
    img = img_as_float(img)
    bg = rescale(img, 1/scale, order=1)
    bg = rolling_ball(bg, radius=radius, nansafe=False)
    bg = rescale(bg, scale, order=1)
    # bg = uniform_filter(bg, size=3)
    return img_as_ubyte(img-bg)

Ref: scikit