Put what is done by `autfilt` in a new function, `decompose_acc_scc`.
* bin/autfilt.cc: Move code from here...
* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: To here.
* tests/python/decompose_scc.py: Test python binding.
See #172.
While at it, fix typo in doxygen comment.
* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: New function.
* tests/python/decompose_scc.py, tests/Makefile.am: Test python
binding.
* spot/twaalgos/mask.hh: Fix typo.
