improve partial_degeneralize() on several cases

On these 4 cases, added to pdegen.py, and supplied by Florian Renkin,
partial_degeneralize() is now at least as good as degeneralize_tba(),
and sometimes better.  This is achieved as follows: (1) a
propagate_marks procedure is introduced to propagate marks as far as
possible on the automaton (e.g., common outgoing marks can be push
onto the incoming transitions and vice-versa), (2) the
degeneralization order is compute dynamically, and (3) whenever and
fully-accepting transition is taken in the original automaton, the
destination level is chosen to be the highest existing level.

* spot/twaalgos/degen.cc,
spot/twaalgos/degen.hh (propagate_marks_vector, propagate_marks_here):
New functions.
(partial_degeneralize): Improve, as described above.
* tests/python/pdegen.py: Add test cases.

NEWS

@@ -58,7 +58,7 @@ New in spot 2.8.5.dev (not yet released)
        parity_min_even(n) = parity_min(false, n)
 
   - partial_degeneralize() is a new function performing partial
-    degeneralization to get rid of conjunction of Inf terms in
+    degeneralization to get rid of conjunctions of Inf terms in
     acceptance conditions.
 
   - simplify_acceptance_here() and simplify_acceptance() learned to
@@ -68,6 +68,16 @@ New in spot 2.8.5.dev (not yet released)
     and the automaton is adjusted to that i also appears where j
     appeared.
 
+  - propagate_marks_vector() and propagate_marks_here() are helper
+    functions for propagatings marks on the automaton: ignoring
+    self-loop and out-of-SCC transitions, marks common to all the
+    input transitions of a state can be pushed to all outgoing
+    transitions of a state, and vice-versa.  This is repeated until a
+    fix point is reached.  propagate_marks_vector() does not modify
+    the automaton and returns a vector of the acc_cond::mark_t that
+    should be on each transition; propagate_marks_here() actually
+    modifies the automaton.
+
 New in spot 2.8.5 (2020-01-04)
 
   Bugs fixed: