Added signed option when saving JPEG2000 images

[radarhere]

Resolves #5502

Allows

im.save(outfile, signed=True)

to save JPEG2000 sgnd images.

[hugovk]

Shall we do it as a keyword-only argument? So it can be called like:

im.save(outfile)
im.save(outfile, signed=True)
im.save(outfile, signed=False)

But not:

im.save(outfile, True)
im.save(outfile, False)

That should make things clearer, no need to wonder what True or False mean here.

[hugovk]

And let's add this to the release notes.

[radarhere]

It should already be a keyword-only argument, as is the rule for encoderinfo keys.

I've added release notes.

[hugovk]

Thanks!