def add_column(name, column_properties = {})
      validate_column_absence(name)
      create_column_properties(name, column_properties)
      @data_rows.each{|x|
        x << (yield Row.new(@column_indexes, x))
      }
      @column_indexes[name] = @column_indexes.size
      self # enable chaining
    end
def add_column(name, column_properties = {})
      validate_column_absence(name)
      create_column_properties(name, column_properties)
      @data_rows.each{|x|
        x << (yield Row.new(@column_indexes, x))
      }
      @column_indexes[name] = @column_indexes.size
      self # enable chaining
    end
adds a column with given name to the far right of the table
@throws if given column name already exists