Need a low overhead way to get worksheetNames and specify which sheetNames to process

[cboleary]

when ReadData is called using parseXLSX, it blindly processes every worksheet in the workbook.
This can take several minutes with a very large spreadsheet with many workbooks

It would be good to have a way to

1. just ask for just the worksheet names with minimum processing
   and
2. have a way to tell it to only process worksheets specified in a list

I have crudely implemented these changes in pull request #98