Function add_sheet 

pub fn add_sheet(
    workbook_xml: &mut WorkbookXml,
    workbook_rels: &mut Relationships,
    content_types: &mut ContentTypes,
    worksheets: &mut Vec<(String, WorksheetXml)>,
    name: &str,
    worksheet_data: WorksheetXml,
) -> Result<usize>

Add a new sheet. Returns the 0-based index of the new sheet.

This function performs all bookkeeping: adds entries to the sheet list, workbook relationships, and content type overrides.