def to_h
      keys.map { |k| [k, map { |h| h[k] }] }.to_h
    end
def to_h
      keys.map { |k| [k, map { |h| h[k] }] }.to_h
    end
Represents DataTable as a `column name => all values in columns`
 hash.

 @return [Hash{String => Array}]