Pivot tables: colFields "data" field always after user specified colFields #2425

// addPivotColFields create pivot column fields by given pivot table
// definition and option.
func (f *File) addPivotColFields(pt *xlsxPivotTableDefinition, opts *PivotTableOptions) error {
	if len(opts.Columns) == 0 {
		if len(opts.Data) <= 1 {
			return nil
		}
		pt.ColFields = &xlsxColFields{}
		// in order to create pivot table in case there is no input from Columns
		pt.ColFields.Count = 1
		pt.ColFields.Field = append(pt.ColFields.Field, &xlsxField{
			X: -2,
		})
		return nil
	}

	pt.ColFields = &xlsxColFields{}

	// col fields
	colFieldsIndex, err := f.getPivotFieldsIndex(opts.Columns, opts)
	if err != nil {
		return err
	}
	for _, fieldIdx := range colFieldsIndex {
		pt.ColFields.Field = append(pt.ColFields.Field, &xlsxField{
			X: fieldIdx, // here is user defined colFields always before DATA colField (-2) and no opportunity to change order
		})
	}

	// in order to create pivot in case there is many Columns and Data
	if len(opts.Data) > 1 {
		pt.ColFields.Field = append(pt.ColFields.Field, &xlsxField{
			X: -2, // here is -2 always after user defined fields and no opportunity to change order 
		})
	} 

	// count col fields
	pt.ColFields.Count = len(pt.ColFields.Field)
	return err
}