This implements a format guessing technique that looks at all cells in
a column to figure things out.  It is not particular smart, but it does
handle...

1. Dates.  Given enough dates, the field order will be deduced.

2. Numbers.  This will figure out if decimal point or decimal comma
   is being used.  This is not always possible, for example if all
   numbers are of the forms [-+]?\d+ and [-+]?\d{1,3},\d{3}