* spot/tl/contain.cc, spot/tl/contain.hh: Simplify the
use of language_containment_checker by adding default argument.
* python/spot/__init__.py, python/spot/impl.i: Bind it in Python.
* doc/org/tut04.org: New file to illustrate it.
* doc/org/tut.org, doc/Makefile.am: Add it.
* NEWS: Mention those changes.
