genltl: add support for --sejk-f=n,m

Together with the previous patch, this Fixes #353.

Implementing this required to extend our interface two support
two-parameter patterns.

* spot/gen/formulas.cc, spot/gen/formulas.hh: Implement it.
* bin/genltl.cc: Add --sejk-f.
* bin/common_output.cc, bin/common_output.hh: Adjust to handle
"line numbers" that are not integers (e.g., "3,2"), since those
are used to display pattern parameters.
* bin/ltlfilt.cc: Adjust.
* python/spot/gen.i: Add support for two-parameters patterns.
* tests/core/genltl.test, tests/python/gen.ipynb: Augment.
* NEWS: Mention it.

NEWS

@@ -22,7 +22,9 @@ New in spot 2.5.3.dev  (not yet released)
     product to avoid exceeding the number of supported
     acceptance sets.
 
-  - genltl learned to generate three new families of formulas:
+  - genltl learned to generate four new families of formulas:
+      --sejk-f=RANGE[,RANGE] f(0,j)=(GFa0 U X^j(b)), f(i,j)=(GFai U
+                             G(f(i-1,j)))
       --sejk-j=RANGE         (GFa1&...&GFan) -> (GFb1&...&GFbn)
       --sejk-k=RANGE         (GFa1|FGb1)&...&(GFan|FGbn)
       --sejk-patterns[=RANGE]   φ₁,φ₂,φ₃ from Sikert et al's [CAV'16]