Cor adaptive search in selected regions

implements Benoit suggestions. An additional optional parameter is introduced in find_shift : guess It can be a tuple of len 2 , a tuple of len 1, or a scalar. The first scalar is taken as the guess for the CoR position in units of pixels from the center of the image. The second scalar, when given is the radius of the search area. If not given the radius is taken as on tenth of the image dimension