EVL XLS hyphens and blank spaces in numeric fields

The EVL spreadsheet contains some blank cells and hyphens. If we store these
as strings we can't do maths on them. They should be zeros.

Makes the test data closer to the real data.
Merge pull request #145 from alphagov/bugfix-evl-dodgy-spreadsheet

EVL XLS: handle hyphens and blank spaces in numeric fields