print_hoa: add support for universal branches

* spot/twaalgos/hoa.cc: Implement it.
* tests/python/alternating.py: Test it.

tests/python/alternating.py

@@ -50,3 +50,22 @@ try:
 except RuntimeError:
     received = True
 assert received
+
+h = aut.to_str('hoa')
+print(h)
+assert h == """HOA: v1
+States: 3
+Start: 0
+AP: 2 "p1" "p2"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: univ-branch trans-labels explicit-labels trans-acc
+--BODY--
+State: 0
+[0] 1&2 {0}
+[1] 0&1
+State: 1
+[0&1] 0&2&1
+State: 2
+[0 | 1] 2
+--END--"""