Adds an `mv_join` function that joins together multivalue string fields.
You can combine this with out fancy new `to_string` to join together any
multivalued fields into a string.
This adds a `mv_median` function that converts a multivalued field into
a single valued field by picking the median. If there are an even number
of values we return the average of the middle two numbers. If the input
type is `int` or `long` then the average rounds *down*.
Nik Everett
Abdon Pijpelink