
there are some issues with the imregister database that should get
addressed eventually.  currently, I am loading and saving the entire
file each time I make a change.  this seems like a bad plan.  when we
get a few hundred nights of images in there, we will have several
megabytes to load, change, and save each time.  two things could
change.  first, imregister (imsort) could only add a record at the end
of the file.  second, the imstatreg function could find the right
image and write over the few needed bytes.

should I uses mmap to do the I/O?

