This also fixes error handling for malformed dates

It is now possible to specify a date format string (as used by
strptime) for date columns in the spreadsheet.

Still need to handle row numbers correctly

Includes a hacky fix to skip ignored values and a temporary hardcoding
of the sheet profile.

normalize_row doesn't really need to dealw ith None
and theres a slightly more elegant way to calculate row numbers

Google CSV to XLSX conversion results in dates being stored as date objects
rather than strings, so we need to cope with these