code: def add_field(self, name, data):
          
          if name in self._fields:
              raise ValueError
          if not len(data) == self._num_fix:
              raise ValueError
          self._fields.append(name)
          self.__dict__[name] = data
docs: Add a new field to the datamat.
  
          Parameters:
              name : string
                  Name of the new field
              data : list
                  Data for the new field, must be same length as all other fields.