Implement an accepting run search in spot::sccinfo, use it with the
generic emptiness check.
* spot/twaalgos/sccinfo.cc, spot/twaalgos/sccinfo.hh: Here.
* spot/twaalgos/genem.cc, spot/twaalgos/genem.hh: Use it.
* tests/python/genem.py: Test it.
Fixes#360.
* spot/twa/acc.cc, spot/twa/acc.hh (force_inf): New method.
* spot/twaalgos/genem.cc: Fix the emptiness check using force_inf.
* tests/python/genem.py: Add test case and adjust the python
version of the emptiness check.
* spot/twa/acc.cc, spot/twa/acc.hh (fin_unit, one_fin): New function.
* spot/twaalgos/genem.cc, spot/twaalgos/genem.hh: New files.
* spot/twaalgos/Makefile.am: Add it.
* tests/python/genem.py: New file.
* tests/Makefile.am: Add it.
* python/spot/impl.i: Add bindings for genem.hh.
* NEWS: Mention the new function.
