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.

NEWS

@@ -12,11 +12,23 @@ New in spot 1.99.7a (not yet released)
     object can be queried about the supported variables and their
     types.
 
+  * print_dot() now accepts an option of the form "<N" to specify
+    a maximum number of states to output.  Incomplete states are
+    then marked appropriately.  For a quick example, compare
+       ltl2tgba -D 'Ga | Gb | Gc' -d'<3'
+    with
+       ltl2tgba -D 'Ga | Gb | Gc' -d
+
   Python:
 
   * The ltsmin interface has been binded in Python.  See
     https://spot.lrde.epita.fr/ipynb/ltsmin.html for a short example.
 
+  * spot.setup() sets de maximum number of states to display in
+    automata to 50 by default, as more states is likely to be
+    unreadable (and slow to process by GraphViz).  This can be
+    overridden by calling spot.setup(max_states=N).
+
   Documentation:
 
   * There is a new page giving informal illustrations (and extra