* spot/twaalgos/degen.hh, spot/twaalgos/degen.cc: Adjust
degeneralize() and degeneralize_tba() to work on generalized-co-Büchi.
* NEWS: Mention this.
* spot/twaalgos/cobuchi.hh, spot/twaalgos/cobuchi.cc (to_nca): Use
degeneralization on generalized-co-Büchi.
* spot/twaalgos/postproc.cc: Use degeneralization for generalized
co-Büchi as well.
* bin/autfilt.cc: Improve chain products of co-Büchi automata by using
generalization if too many colors are needed.
* tests/core/prodchain.test, tests/python/pdegen.py: Add test cases.
Improve the construction of the above constructions, saving colors.
* spot/twaalgos/product.cc: Here.
* spot/twaalgos/product.hh, NEWS: Mention it.
* tests/core/prodchain.test, tests/core/prodor.test,
tests/python/_product_weak.ipynb: Adjust.
Fixes#348, reported by Jeroen Meijer.
* bin/autfilt.cc: If -B is used with many --product,
degeneralize intermediate products as needed.
* NEWS: Mention the change.
* tests/core/prodchain.test: New file.
* tests/Makefile.am: Add it.
* spot/twa/acc.cc, spot/twa/acc.hh: Fix reporting of
overflow.
* tests/core/acc.cc: Adjust.