more documentation for twa_graph internals

* spot/graph/graph.hh, spot/twa/twagraph.hh, spot/twa/twagraph.cc:
Implement a dump_storage_as_dot() method.
* python/spot/__init__.py (twa_graph.show_storage): New method, above
dump_storage_as_dot().
* tests/python/twagraph-internals.ipynb: New file, with documentation
about the twa_graph internals, using show_storage() to illustrate
everything.
* tests/Makefile.am, doc/org/tut.org: Add it.
* python/spot/impl.i: Add bindings for out_iterasor, demonstrated in
the Python notebook.
* spot/twa/twa.hh: Add prop_reset().  Used in the notebook.
* NEWS: Mention the new notebook and function.
* doc/org/tut50.org: Link to the notebook.
* tests/python/ipnbdoctest.py: Adjust for twa_graph_ptr being
redefined in the spot namespace.

python/spot/__init__.py

@@ -175,6 +175,13 @@ class twa:
                 f.write('\n')
         return a
 
+@_extend(twa_graph)
+class twa_graph:
+    def show_storage(self, opt=None):
+        ostr = ostringstream()
+        self.dump_storage_as_dot(ostr, opt)
+        from IPython.display import SVG
+        return SVG(_ostream_to_svg(ostr))
 
 @_extend(formula)
 class formula:
@@ -761,6 +768,7 @@ def postprocess(automaton, *args, formula=None):
 
 twa.postprocess = postprocess
 
+
 # Wrap C++-functions into lambdas so that they get converted into
 # instance methods (i.e., self passed as first argument
 # automatically), because only user-defined functions are converted as