missing file from last commit!  oops.
parallel: add new method to broadcast booleans.
netz: it works!  correctly filtering out particular field.

basically we keep track of where we are in the output; if the
desired field is also there, we push it out.

also started using my debug code here too.. if the code for the
NETZ guys is going to live in this repo, it might as well use it.
but we should probably move the code to be separate at some
point...
add forgotten file that's been missing forever.

been bad at this lately...
write out separate files for each field they want.

instead of just a giant file with everything.
netz: rebuild overall domain from pieces

right now, this unfortunately just waits until each brick is
written to disk, and then re-reads it and writes out the
appropriate file.  kind of misses the point, but it's a lot easier
&& the final solution will need to look a lot different anyway.