Correctly open Excel files with the right dimensions

[RickMoynihan]

Excel files like this one:

SttsNZ-Pivot-2.xlsx

are misinterpreted by Grafters Datasets. Grafter will incorrectly load this as containing one or two columns (I forget which), because it assumes Datasets have the same width as their first row.

It seems Apache POI has a method to get the Dimensions of the sheet, we see whether we can use this. Or perhaps modify clj-excel to support it:

http://stackoverflow.com/questions/5778599/how-to-get-the-max-no-of-columns-filled-in-an-xlsx-file-using-poi

A similar issue #46 exists in CSV files, if the first row isn't as wide as the widest row.