create-template: Output XLSX worksheets in schema order

[duncandewhurst]

Presently, the create-template command outputs XLSX worksheets in alphabetical order by worksheet name, with the exception of the main sheet.

From GFDRR/rdls-spreadsheet-template#1, it would be preferable for the worksheets to be output in schema order so that users can complete them in a logical order.

Edit: For RDLS, we can workaround this with a post-processing step, for now.