product: fix handling of output_aborter

* spot/twaalgos/product.cc: The res pointer should be
passed by reference since we reset it to nullptr when
output_aborter says "too large".
* python/spot/impl.i: Add binding for powerset.hh,
so we can use output_aborter in Python.
* tests/python/prodexpt.py: Test it.
16 jobs for next in 318 minutes and 23 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #53833
alpine-gcc

00:27:06

passed #53834
arch-clang

00:50:36

passed #53846
arch-gcc-glibcxxdebug

00:42:10

passed #53832
debian-gcc-snapshot

01:55:30

passed #53830
debian-stable-gcc

00:50:54

passed #53831
debian-unstable-gcc-coverage

00:46:18

90.1%
passed #53848
debpkg-stable

01:51:33

passed #53847
debpkg-unstable

03:14:21

passed #53836
mingw-shared

00:09:36

passed #53837
mingw-static

00:11:39

passed #53840
rpm-pkg

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failed #53835
arch-gcc-glibcxxdebug

01:14:44

failed #53838
debpkg-stable

01:30:45

failed #53839
debpkg-unstable

01:30:44

 
  Publish
passed #53841
dput
publish-rpm

00:00:44

passed #53842
dput
publish-unstable

00:02:35