Function copy_sheet 

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

Copy a sheet, returning the 0-based index of the new copy.