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.

bin/ltlfilt.cc

@@ -781,7 +781,8 @@ namespace
               for (auto& p: m)
                 stream_formula(opt->output_define->ostream()
                                << "#define " << p.first << " (",
-                               p.second, filename, linenum) << ")\n";
+                               p.second, filename,
+                               std::to_string(linenum).c_str()) << ")\n";
             }
           one_match = true;
           output_formula_checked(f, &timer, filename, linenum, prefix, suffix);