introduce containement checks functions

* 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.

NEWS

@@ -60,6 +60,11 @@ New in spot 2.5.3.dev  (not yet released)
     options; there are demonstrated on the new
     https://spot.lrde.epita.fr/ipynb/satmin.html page.
 
+  - spot::contains() and spot::are_equivalent() can be used to
+    check language containement between two automata or formulas.
+    They are most welcome in Python, since we used to redefine
+    them every now and them.
+
   Python:
 
   - New spot.jupyter package.  This currently contains a function for