We have a couple boost::variant typedefs that might be able to be merged so that the code that handles their conversion could be unified a bit (mostly an issue in python bindings).
main difference is that value_holder uses std::string but mapnik::value only uses UnicodeString
python: refactor interface to mapnik::parameters using poor man's ind…
…exingapproach - long term todo is merge mapnik::value_holder and mapnik::value to make this cleaner - refs #976
not actionable. more or less fixed by #1177