Follow-up to an email from Ayrat Khalimov.
* python/spot/impl.i: Include twa/formula2bdd.hh.
* python/spot/__init__.py: Make the dictionnary
optional.
* spot/twa/formula2bdd.cc: Throw an exception instead of asserting.
* tests/python/bdditer.py: New file.
* tests/Makefile.am: Add it.
* NEWS: Update.
