ODS output

[rowanhemsi]

Is your feature request related to a problem? Please describe.
gov.uk requires spreadsheets in ODS format. Currently gptables only outputs XLSX files.

Describe the solution you'd like
Option when using write_workbook to change output file format

Additional context
Underlying xlsxwriter package only writes to XLSX - would likely require writing to XLSX, reading and rewriting to ODS

[rowanhemsi]

pyexcel may offer a solution, but need to test whether essential accessibility criteria are correctly converted

[rowanhemsi]

(First mentioned in #143)

[josephelliston]

I second this feature request - as it currently takes much time for us to manually convert XLSX to ODS

[giro090]

Definition of Done:

• Files can be saved as .ods files, with formatting preserved correctly.