This allows each class to define the type of data that they return,
regardless of the type of column to which they are applied. This is the
pre-cursor to work on graphing and filtering of aggregated data.
E.g. a text field that is aggregated with "count" is returning numeric
data rather than text.
The group concatenation types now consistently sort the returned values
and support aggregating columns containing multiple fields and those
with defined callbacks.
Paul Holden