Fixes #350.

* spot/twaalgos/product.cc: Implement this change.
* NEWS, spot/twaalgos/product.hh: Mention it.
* spot/twa/acc.cc, spot/twa/acc.hh (acc_cond::sat_mark): New method.
* tests/python/_product_weak.ipynb: New file.
* tests/Makefile.am: Add it.
* tests/python/automata.ipynb, tests/python/highlighting.ipynb,
tests/python/product.ipynb, tests/core/prodor.test: Adjust test cases.