Gradient search add missing data on the edges

[mraspaud]

The upper left panel is a resampled B03 image (2000 * 2000). The right and bottom panels show the missing values ​​in the right 5 columns and the bottom 5 rows, respectively, as yellow lines.

Missing values ​​are difficult to see due to the large number of pixels in the satellite image, but can be seen well when magnified.

I use it after understanding that it is experimental. Thank you.

Originally posted by @tsukada-cs in https://github.com/pytroll/satpy/issues/1039#issuecomment-572455355