Change in handling of image border regions. These are notoriously problematic, since it is not clear what the correct charge deflections there should look like.
One could:
* do all calculable shifts, i.e. only ignore deflections due to charges in pixels that are outside the Image
  [this is what I did so far, but it is not charge conserving unless the image is flat]
* use periodic boundary conditions
  [this is what a straightforward Fourier implementation would do, but it's clearly not physical]
* do not shift pixel borders that lie on the image border
  [this would cause glowing edges I think, which may or may not be what we want, depending on whether we simulate postage stamps or full CCDs worth of data]
* only apply shifts due to charges where the pixel mirrored on the respective pixel border is inside the image, too
  [this is a parity symmetry of the model, and I've decided to enforce it in the implementation as of this commit; it's charge conserving and leaves flats invariant]
* (there may be more options)

Also changed test to float64 and increased accuracy limits. @barnabytrowe -- could you check whether these work on your system now, too, and whether the limits make more sense?

(#524)