def columns(*names)
      names.map!(&:to_s)
      DataTable.new(map { |h| names.map { |n| [n, h[n]] }.to_h })
    end
def columns(*names)
      names.map!(&:to_s)
      DataTable.new(map { |h| names.map { |n| [n, h[n]] }.to_h })
    end
Slice of a DataTable with only specified columns left.

 @param names [Array<String>] What columns to leave in a DataTable
 @return [DataTable]