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.

doc/org/tut.org

@@ -85,3 +85,4 @@ real notebooks instead.
 - [[https://spot.lrde.epita.fr/ipynb/alternation.html][=alternation.ipynb=]] examples of alternating automata.
 - [[https://spot.lrde.epita.fr/ipynb/stutter-inv.html][=stutter-inv.ipynb=]] working with stutter-invariant formulas properties.
 - [[https://spot.lrde.epita.fr/ipynb/satmin.html][=satmin.ipynb=]] Python interface for [[file:satmin.org][SAT-based minimization of deterministic ω-automata]].
+- [[https://spot.lrde.epita.fr/ipynb/twagraph-internals.html][=twagraph-internals.ipynb=]] Inner workings of the =twa_graph= class.

doc/org/tut50.org

@@ -21,9 +21,10 @@ from the initial state of an automaton.
 
 The explicit interface can only be used on =twa_graph= objects.  In
 this interface, states and edges are referred to by numbers that are
-indices into state and edge vectors.  This interface is lightweight,
-and is the preferred interface for writing most automata algorithms in
-Spot.
+indices into state and edge vectors (the [[https://spot.lrde.epita.fr/ipynb/satmin.html][=twagraph-internals.ipynb=]]
+notebook shows these vectors graphically).  This interface is
+lightweight, and is the preferred interface for writing most automata
+algorithms in Spot.
 
 ** How this interface works