The ZipHelper interface is now more generic and allow single files to be added.
It supports adding uncompressed files (for PHP7+), which is required to have the mime detection magic work with ODS files.
Also fixed a few issues with the created ODS file (thanks to https://odf-validator.rhcloud.com/)
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
