add serial processor to inpainter
shift valid x region
split up long invalid slices

In some runs there is a contiguous region of nans that has a long time
extent. It takes a very long time to construct the interpolator for such
regions because of the number of points. Here we set a maximum time
extent for the slice size and split big slices up into a number of
smaller ones.
single layer process switch
pin outer faces rather than corners
adjustable interpolation recursion depth
construct minimal valid points in generator
parallel process with explicit control
use flush writer by default for shared progress
put number done in progressbar
todo