add validation check to loop_in_chunks

[stevengj]

For things like #1895, it might be good to add a validation check to the loop_in_chunks code: as we loop over the chunks, keep a running sum of the total number of grid points in all of the chunkloop calls, and then check at the end that this total equals the number of points in the original volume. (If not, emit a warning.)

I think this should be cheap, so it wouldn't hurt to do it all the time. But if we want we could only enable it in --enable-debug mode.

cc @mochen4, @mawc2019