introduce count_univbranch_states() and count_univbranch_edges()

Fixes #368, suggested by František Blahoudek.

* spot/twaalgos/isdet.cc, spot/twaalgos/isdet.hh
(count_univbranch_states(), count_univbranch_edges()): New functions.
* bin/common_aoutput.cc, bin/common_aoutput.hh: Add %u and variants.
* NEWS: Mention these.
* tests/core/alternating.test: Test them.

NEWS

@@ -18,6 +18,11 @@ New in spot 2.6.3.dev (not yet released)
   - ltlfilt learned --liveness to match formulas representing liveness
     properties.
 
+  - the --stats= option of tools producing automata learned how to
+    tell if an automaton uses universal branching (%u), or more
+    precisely how many states (%[s]u) or edges (%[e]u) use universal
+    branching.
+
   Python:
 
   - spot.translate() and spot.postprocess() now take an xargs=
@@ -75,6 +80,10 @@ New in spot 2.6.3.dev (not yet released)
     used to check whether a formula or an automaton represents a
     liveness property.
 
+  - Two new functions count_univbranch_states() and
+    count_univbranch_edges() can help measuring the amount of
+    universal branching in alternating automata.
+
   Bugs fixed:
 
   - The pair of acc_cond::mark_t returned by