Last sheet in Excel workbook is often blank

Datapusher selects the LAST sheet in an Excel workbook as the one to push. However according to [this user](https://stackoverflow.com/questions/33764220/error-uploading-xls-or-xlsx-files-to-datastore?noredirect=1#comment55334985_33764220) Excel adds 3 blank sheets to a new workbook, so in most cases the last sheet will be blank. This has caused grief.

I imagine that the last sheet was picked for cases where the first sheet is more of an index to the rest of the sheet - ONS uses this in the UK.

So it sounds like a bit of logic is needed to find the last sheet which is not empty.