/// <summary>
        /// Loads the Purchase Report and filters the columns by column headers(by opening a ColumnHeaderSelectForm)
        /// </summary>
        private void LoadPurchaseReportButton_Click(object sender, EventArgs e)
        {
            excelReportContent = excelReader.GetExcelContent()[0];

            List <string> documentHeaders = new List <string>();

            for (int i = 0; i < excelReportContent.Columns.Count; i++)
            {
                documentHeaders.Add(excelReportContent.Columns[i].ColumnName);
            }

            columnHeaderSelectForm = new ColumnHeaderSelectForm(this, documentHeaders, DocumentType.PurchaseReport, docHandler);
            columnHeaderSelectForm.Show();
        }
        /// <summary>
        /// Loads the Sales Invoice and filters the columns by column headers(by opening a ColumnHeaderSelectForm)
        /// </summary>
        private void LoadSalesInvoiceButton_Click(object sender, EventArgs e)
        {
            excelInvoiceContent = excelReader.GetExcelContent()[0];

            List <string> invoiceHeaders = new List <string>();

            for (int i = 0; i < excelInvoiceContent.Columns.Count; i++)
            {
                invoiceHeaders.Add(excelInvoiceContent.Columns[i].ColumnName);
            }

            columnHeaderSelectForm = new ColumnHeaderSelectForm(this, invoiceHeaders, DocumentType.SalesInvoice, docHandler);
            columnHeaderSelectForm.Show();
        }