* spot/tl/Makefile.am: New sonf files
* spot/tl/sonf.cc,spot/tl/sonf.hh: Here.
* python/spot/impl.i: include sonf.hh header
* doc/spot.bib: add entry for the SONF paper
* tests/python/formulas.ipynb: show sample usage
* tests/python/spot.py: test automata equivalence before/after SONF
* NEWS: mention the change
