An aggregate field represents a field's aggregated values, either {@link ColumnField} or {@link FormulaField}. It also may be associated with a group (assigned in {@link ReportReader#field} or when editing a report),meaning that the aggregate value is reset whenever the group's value changes. The value of an aggregate field holds the id of some other field whose value we are aggregating.
@author Jim Menard,
jimm@io.com