postproc: introduce --parity output

* spot/twaalgos/postproc.hh, spot/twaalgos/postproc.cc: Add parity
  options.
* bin/common_post.cc: Add support for --parity.
* NEWS: Mention it.
* tests/core/parity2.test: New file.
* tests/Makefile.am: Add it.

NEWS

@@ -47,6 +47,11 @@ New in spot 2.4.4.dev (net yet released)
   - ltlsynt is a new tool for synthesizing a controller from LTL/PSL
     specifications.
 
+  - ltl2tgba, autfilt, and dstar2tgba have a new '--parity' option to
+    force parity acceptance on the output.  Different styles can be
+    requested using for instance --parity='min odd' or --parity='max
+    even'.
+
   - ltldo learned to limit the number of automata it outputs using -n.
 
   - autcross, ltlcross, and ltldo learned --fail-on-timeout.
@@ -176,6 +181,11 @@ New in spot 2.4.4.dev (net yet released)
     optimization for stutter-invariant automata that may produce slightly
     smaller automata.
 
+  - spot::postprocessor::set_type() can now request different forms of
+    parity acceptance as output.  However currently the conversions
+    are not very smart: if the input does not already have parity
+    acceptance, it will simply be degeneralized or determinized.
+
   - acc_cond::name(fmt) is a new method that name well-known acceptance
     conditions.   The fmt parameter specify the format to use for that
     name (e.g. to the style used in HOA, or that used by print_dot()).