def format_column idx, format=nil, opts={}
      opts[:worksheet] = self
      res = case idx
            when Integer
              column = nil
              if format
                column = Column.new(idx, format, opts)
              end
              @columns[idx] = column
            else
              idx.collect do |col| format_column col, format, opts end
            end
      shorten @columns
      res
    end
def format_column idx, format=nil, opts={}
      opts[:worksheet] = self
      res = case idx
            when Integer
              column = nil
              if format
                column = Column.new(idx, format, opts)
              end
              @columns[idx] = column
            else
              idx.collect do |col| format_column col, format, opts end
            end
      shorten @columns
      res
    end
Sets the default Format of the column at _idx_.

 _idx_ may be an Integer, or an Enumerable that iterates over a number of
 Integers.

 _format_ is a Format, or nil if you want to remove the Formatting at _idx_

 Returns an instance of Column if _idx_ is an Integer, an Array of Columns
 otherwise.