Add documentation for lateral_fill with use_sor=True

[mnlevy1981]

I was thinking it might be nice to add a plot to the lateral_fill documentation showing what the fill looks like if we set use_sor=True.

If we do add this, I was envisioning a three-panel plot in the Fill the Missing Values section that shows the original fill, the SOR fill, and the difference between the two:

It might also be useful to have %time before the two fill calls to see the difference in runtime (though that may not be too useful on the gx3v7 grid).