introduce spot::split_edges()

Fixes #255.

* spot/twaalgos/split.cc, spot/twaalgos/split.hh,
tests/core/split.test: New files.
* spot/twaalgos/Makefile.am, tests/Makefile.am: Add them.
* bin/autfilt.cc (--split-edges): New option.
* python/spot/impl.i: Process split.hh.
* tests/python/alternating.py: Test split_edges() on
an alternating automaton.

NEWS

@@ -14,6 +14,10 @@ New in spot 2.3.3.dev (not yet released)
   - genaut is a binary to produce families of automata defined in the
     literature (in the same way as we have genltl for LTL formulas).
 
+  - autfilt learned --split-edges to convert labels that are Boolean
+    formulas into labels that are min-terms.  (See spot::split_edges()
+    below.)
+
   Library:
 
   - A new library, libspotgen, gathers all functions used to generate
@@ -62,6 +66,13 @@ New in spot 2.3.3.dev (not yet released)
     level.  A similar trick was already used in sbacc(), and saves a
     few states in some cases.
 
+  - There is a new spot::split_edges() function that transforms edges
+    (labeled by Boolean formulas over atomic propositions) into
+    transitions (labeled by conjunctions where each atomic proposition
+    appear either positive or negative).  This can be used to
+    preprocess automata before feeding them to algorithms or tools
+    that expect transitions labeled by letters.
+
   Python:
 
   - The 'spot.gen' package exports the functions from libspotgen.