Changes to Dullard::Sheet xml file reference

- Now finds the corresponding xl/worksheets/sheet#.xml file based on the index position in xl/workbook.xml instead of the sheetId.
- This situation came about when I had a workbook with two sheets (sheetId 9 and 8) with corresponding xml files named sheet1.xml and sheet2.xml.
- It also checks if the file exists before attempting to parse it.

Can you also provide SHAPE5_CorePrePost.xlsx to ensure that the specs pass?