org: add a first code example

The difficulty is not the example, but setting up org-mode to allow
Python and C++ example that use the local libraries, not those
installed system-wide.

* doc/org/.dir-locals.el: Rename as...
* doc/org/.dir-locals.el.in: ... this, so we can easily define
PYTHONPATH and other environment variables.
* doc/org/init.el.in: Enable C++, and make sure but Python
and C++ use the local libraries.
* doc/org/g++wrap.in, doc/org/tut01.org: New files.
* doc/Makefile.am, configure.ac: Adjust.
* wrap/python/spot.py (to_str): Take a parenth argument.

doc/Makefile.am

@@ -57,7 +57,8 @@ org:
 	rm -rf tmp
 
 ORG_FILES = \
-  org/.dir-locals.el \
+  org/.dir-locals.el.in \
+  org/g++wrap.in \
   org/init.el.in \
   org/syntax.css \
   org/spot.css \
@@ -77,6 +78,7 @@ ORG_FILES = \
   org/randaut.org \
   org/randltl.org \
   org/tools.org \
+  org/tut01.org \
   org/satmin.org \
   org/satmin.tex \
   org/setup.org \