def columns(*names)
      columns = @base.columns.select {|c| names.include?(c.name) }
      check_columns_subset_of_base_dimension names, columns
      @options[:columns] = columns
    end
def columns(*names)
      columns = @base.columns.select {|c| names.include?(c.name) }
      check_columns_subset_of_base_dimension names, columns
      @options[:columns] = columns
    end
Defines which columns of the base dimension are present in the
 shrunken dimension.

 Takes an array of the column names as symbols.

 The columns must be a subset of the base dimension's columns;
 additional names will raise a Chicago::MissingDefinitionError.