dot: extend the palette from 9 to 16 colors

Let's close #212 even if this does not cover the 32 sets.

* spot/twaalgos/dot.cc: Change the palette.
* doc/org/autfilt.org, NEWS: Adjust documentation.
* tests/core/alternating.test, tests/core/readsave.test,
tests/core/tgbagraph.test, tests/python/_altscc.ipynb,
tests/python/alternation.ipynb, tests/python/atva16-fig2a.ipynb,
tests/python/automata-io.ipynb, tests/python/automata.ipynb,
tests/python/decompose.ipynb, tests/python/gen.ipynb,
tests/python/highlighting.ipynb, tests/python/ltsmin-dve.ipynb,
tests/python/piperead.ipynb, tests/python/product.ipynb,
tests/python/randaut.ipynb, tests/python/word.ipynb: Adjust
test cases.

NEWS

@@ -211,6 +211,10 @@ New in spot 2.3.5.dev (not yet released)
   - spot::acc_cond::mark_t::sets() now returns an internal iterable
     object instead of an std::vector<unsigned>.
 
+  - The color palette optionally used by print_dot() has been extended
+    from 9 to 16 colors.  While the first 8 colors are similar, they
+    are a bit more saturated now.
+
   Deprecation notice:
 
   - spot::decompose_strength() is deprecated, it has been renamed