Remove num-columns-repeated and num-rows-repeated as it does not seem to be required (LibreOffice does not add them).
This greatly simplifies the writer and the XML output.
Added some optional attributes to help LibreOffice with cell values caching ("calcext")
Added ODS writer
Refactored XLSX writer to abstract some pieces into an abstract multi-sheets writer
Created an abstract style helper
Moved shared components around
