Add a new library to generate formulas and automata.

This library, called libspotgen, gathers functions to generate classes
of automata found in the literature.
Related to #254.

* NEWS, README: Mention the modification.
* Makefile.am, debian/control, debian/libspotgen0.install: Build the new
  library in a separate package.
* spot/gen/automata.hh, spot/gen/automata.cc: Add a family of co-Büchi
  automata.
* configure.ac, spot/Makefile.am, spot/gen/Makefile.am: Build the new
  library.

debian/control

@@ -11,7 +11,7 @@ Architecture: any
 Section: science
 Recommends: graphviz
 Suggests: libspot-dev, spot-doc, python3-spot
-Depends: libspot0 (>= ${source:Version}), libspot0 (<< ${source:Version}.1~), ${shlibs:Depends}, ${misc:Depends}
+Depends: ${shlibs:Depends}, ${misc:Depends}, libspot0 (>= ${source:Version}), libspot0 (<< ${source:Version}.1~), libspotgen0 (>= ${source:Version}), libspotgen0 (<< ${source:Version}.1~)
 Description: model checking and omega-automata manipulation library
  Tools to manipulate omega-automata as well as
  linear-time temporal logic (LTL & PSL).
@@ -54,11 +54,19 @@ Description: headers for the BuDDy library
 Package: libspotltsmin0
 Architecture: any
 Section: science
-Recommends: graphviz
 Suggests: libspot-dev
 Depends: ${shlibs:Depends}, ${misc:Depends}, libspot0 (>= ${source:Version}), libspot0 (<< ${source:Version}.1~)
 Description: interface between Spot and LTSmin models
- Library load LTSmin models as Spot automata.
+ Library for loading LTSmin models as Spot automata.
+
+Package: libspotgen0
+Architecture: any
+Section: science
+Suggests: libspot-dev
+Depends: ${shlibs:Depends}, ${misc:Depends}, libspot0 (>= ${source:Version}), libspot0 (<< ${source:Version}.1~)
+Description: functions for generating formulas and automata in Spot
+  Library for generating families of formulas and automata
+  taken from the literature.
 
 Package: spot-doc
 Section: doc