Gets the field name of the column by which data is grouped.
A String value the specifies the column's field name.
When grid data is grouped by the specified column, data rows that have identical values in this column are arranged into groups. Use the FieldName property to obtain the field name of the column by whose values data is grouped. To obtain a column value for which a particular group is created, use the IGroupInfo.Value property. By default, each group row displays a column caption and grouping value separated with colons.