Export features should factor Network Mode more deeply

[dondi]

• For Export Data, for PPI networks, only .sif and .xlsx should be allowed just like with Open, need to check .sif format
• .xlsx export should have the modification to cell A1 of the network sheet, check other options for export (separated out as Modify the name of cell A1 of exporting Excel workbook  #1054)
• Export image and print do not need to be changed
• Unit tests should be expanded to include these cases (separate issue or combine with Modify the name of cell A1 of exporting Excel workbook  #1054?)