Better formula I/O for ipython.

* src/ltlparse/public.hh, src/ltlparse/ltlparse.yy,
src/ltlparse/ltlscan.ll (parse_error): New class.
(parse_formula): New function that raises a parse_error
exception on error.
* src/ltlvisit/tostring.hh, src/ltlvisit/tostring.cc:
(to_sclatex_string): New method.
* wrap/python/spot.i: Catch the parser_error exception,
and use the to_sclatex_string for MathJax rendering.
* wrap/python/tests/run.in: Start ipython on demand.

wrap/python/tests/run.in

@@ -40,6 +40,8 @@ case $1 in
     PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath exec @PYTHON@ "$@";;
   *.test)
     exec sh -x "$@";;
+  ipython*)
+    PYTHONPATH=$pypath DYLD_LIBRARY_PATH=$modpath exec "$@";;
   *)
     echo "Unknown extension" >&2
     exit 2;;