Selection variables don't support negative values

If the "values" parameter of a selection variable includes negative numbers, they get turned into positive numbers. This happens because the values are split up with boost::tokenizer<>, which uses the default (and deprecated) char_delimiters_separator, which seems to treat all punctuation characters as delimiters.

The fix is to use char_separator and explicitly specify the delimiter characters. This leads to the question: Which delimiters should we support? Commas only? Commas and spaces?