def append(data)
      unless data.kind_of? Array
        raise ArgumentError, 'Xcellus.append only accepts Arrays'
      end
      Xcellus::_append(@handle, data.to_json)
    end
def append(data)
      unless data.kind_of? Array
        raise ArgumentError, 'Xcellus.append only accepts Arrays'
      end
      Xcellus::_append(@handle, data.to_json)
    end
Appends sheets and rows to the loaded file. This method expects the same
 structure of Xcellus::transform, with the difference that it creates (when
 necessary) sheets, and appends data to them.