* spot/twaalgos/contains.hh, spot/twaalgos/contains.cc: New files.
* spot/twaalgos/Makefile.am, python/spot/impl.i: Add them.
* python/spot/__init__.py: Also attach these functions as methods,
and support string arguments.
* tests/python/contains.ipynb: New file.
* tests/Makefile.am, doc/org/tut.org: Add it.
* bin/autfilt.cc, tests/python/streett_totgba.py, tests/python/sum.py,
tests/python/toweak.py: Use the new function.
