gen: rename genltl() to ltl_pattern() and introduce ltl_patterns()

* spot/gen/formulas.hh, spot/gen/formulas.cc (genltl): Rename as...
(ltl_pattern): This.
(ltl_pattern_max): New function.
* bin/genltl.cc: Adjust names, and simplify using ltl_pattern_max().
* python/spot/gen.i (ltl_patterns): New function.
* tests/python/gen.py: Test it.
* tests/python/gen.ipynb: New file to document the spot.gen package.
* tests/Makefile.am, doc/org/tut.org: Add gen.ipynb.

python/spot/gen.i

@@ -53,3 +53,36 @@ using namespace spot;
 
 %include <spot/gen/automata.hh>
 %include <spot/gen/formulas.hh>
+
+%pythoncode %{
+def ltl_patterns(*args):
+  """
+  Generate LTL patterns.
+
+  The arguments should be have one of these three forms:
+    - (id, n)
+    - (id, min, max)
+    - id
+  In the first case, the pattern id=n is generated.  In the second
+  case, all pattern id=n for min<=n<=max are generated.  The
+  third case is a shorthand for (id, 1, 10), except when
+  id denotes one of the hard-coded list of LTL formulas (like,
+  DAC_PATTERNS, EH_PATTERNS, etc.) where all formulas from that
+  list are output.
+  """
+  for spec in args:
+    if type(spec) is int:
+      pat = spec
+      min = 1
+      max = ltl_pattern_max(spec) or 10
+    else:
+      ls = len(spec)
+      if ls == 2:
+        pat, min, max = spec[0], spec[1], spec[1]
+      elif ls == 3:
+        pat, min, max = spec
+      else:
+        raise RuntimeError("invalid pattern specification")
+    for n in range(min, max + 1):
+      yield ltl_pattern(pat, n)
+%}