implement maximum cardinality search

* spot/twaalgos/mcs.cc, spot/twaalgos/mcs.hh: New files.
* spot/twaalgos/Makefile.am: Add them.
* python/spot/impl.i: Include mcs.hh.
* bin/autfilt.cc: Add --mcs option.
* NEWS: Mention it.
* doc/spot.bib: Add reference.
* tests/core/mcs.test: New file.
* tests/Makefile.am: Add it.

NEWS

@@ -51,6 +51,10 @@ New in spot 2.12.0.dev (not yet released)
   - autfilt learned --track-formula=F to label states with formulas
     derived from F.  (This is more precise on deterministic automata.)
 
+  - autfilt learned --mcs[=any|scc] to reorder states according to a
+    maximum cardinality search.  The argument specifies how to break
+    ties.
+
   - ltlfilt learned --pi1, --sigma1, --delta1, --pi2, --sigma2, and
     --delta2 to filter according to classes Π₁,Σ₁,Δ₁,Π₂,Σ₂, and Δ₂.
 
@@ -96,6 +100,11 @@ New in spot 2.12.0.dev (not yet released)
   - twa_graph::defrag_states(num) no longer require num[i]≤i; num
     can now describe a permutation of the state numbers.
 
+  - spot::maximum_cardinality_search() and
+    spot::maximum_cardinality_search_reorder_here() are new function
+    to compute (and apply) an ordering of state based on a maximum
+    cardinality search.
+
   Bug fixes:
 
   - Generating random formulas without any unary opertor would very