Add option in GridViewDocumentExportOptions to include / exclude GroupHeaderRowAggregates.

This will allow users to hide/show group header row aggregates in the exported documents.

Option will be globbally applied - to all group header rows in the RadGridView.
===
From 2017 R3 SP GridViewDocumentExportOptions  adds ShowGroupHeaderRowAggregates bool property.

It can be used like so:

  GridViewDocumentExportOptions options = new GridViewDocumentExportOptions()
            {
                ShowGroupHeaderRowAggregates = true
            };
this.gridView.ExportToWorkbook(options);