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/org/.dir-locals.el

@@ -1,37 +0,0 @@
-((nil . ((whitespace-action auto-cleanup)
-	 (whitespace-style face empty trailing lines-tail)
-	 (require-final-newline . t)
-	 (mode . whitespace)))
- (org-mode . ((whitespace-style face empty trailing)
-	      (eval .
-		    (progn
-		      (setq org-babel-sh-command (concat "PATH=../../src/bin"
-							 path-separator
-							 "$PATH sh"))
-		      (setenv "SPOT_DOTDEFAULT" "Brf(Lato)")
-		      (setenv "SPOT_DOTEXTRA" "node[style=filled, fillcolor=\"#ffffa0\"] edge[arrowhead=vee, arrowsize=.7]")
-		      (org-babel-do-load-languages 'org-babel-load-languages
-						   '((sh . t)
-						     (python . t)
-						     (dot . t)))))
-	      (org-confirm-babel-evaluate . nil)
-	      (org-babel-python-command . "/usr/bin/python3")
-	      (org-publish-project-alist
-	       . (("spot-html"
-		   :base-directory "."
-		   :base-extension "org"
-		   :publishing-directory "../userdoc/"
-		   :recursive t
-		   :publishing-function org-publish-org-to-html
-		   ; Org 8
-		   :html-head "<link rel=\"stylesheet\" type=\"text/css\" href=\"spot.css\" />"
-		   ; Org 7
-		   :style "<link rel=\"stylesheet\" type=\"text/css\" href=\"spot.css\" />"
-		   :auto-preamble t)
-		  ("spot-static"
-		   :base-directory "."
-		   :base-extension "css\\|js\\|png\\|jpg\\|gif\\|pdf"
-		   :publishing-directory "../userdoc/"
-		   :recursive t
-		   :publishing-function org-publish-attachment)
-		  ("spot-all" :components ("spot-html" "spot-static")))))))