Updated reader/writer to work with EPPlus 4

Assuming EPPlus has changed it's internals, so we got ```NullReferenceExceptions``` in ```IterateOverRows<T>``` due to the fact that ```ExcelWorksheet worksheet``` seems to have been closed or disposed at that point.

For ```CreateExcelFile<T>```, the issue was that when ```package.Save();``` was called, that closed and disposed the stream we were working on. Now we instead call ```package.SaveAs(outputStream);``` which is a separate stream that we return.