dot: add a <N option

* spot/twaalgos/dot.cc: Implement it.
* spot/taalgos/dot.cc: Ignore it.
* spot/twaalgos/copy.cc, spot/twaalgos/copy.hh: Add option
to limit the number of states.
* tests/python/ltsmin.ipynb: Improve test case.
* tests/Makefile.am: Cleanup the files generated by ltsmin.ipynb.
* python/spot/__init__.py (setup): Add a max_states argument
that default to 50.
* bin/common_aoutput.cc: Mention the <INT option.
* NEWS: Likewise.

spot/twaalgos/copy.hh

@@ -33,5 +33,6 @@ namespace spot
   ///
   /// This works for using the abstract interface for automata
   SPOT_API twa_graph_ptr
-  copy(const const_twa_ptr& aut, twa::prop_set p, bool preserve_names = false);
+  copy(const const_twa_ptr& aut, twa::prop_set p,
+       bool preserve_names = false, unsigned max_states = -1U);
 }