* bin/autfilt.cc: Add a --partial-degeneralize option.
* NEWS: Mention it.
* spot/twaalgos/degen.cc: Do not restrict partial_degeneralize() to
deterministic automata.
* spot/twaalgos/degen.hh: Adjust documentation.
* tests/core/pdegen.test: New test case.
* tests/Makefile.am: Add it.
* tests/python/pdegen.py: Adjust.