<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def by_field(self, field):
 """ Returns an iterator that iterates over unique values of field Parameters: field : string Filters the datamat for every unique value in field and yields the filtered datamat. Returns: datamat : Datamat that is filtered according to one of the unique values in 'field'. """

        for value in np.unique(self.__dict__[field]):
            yield self.filter(self.__dict__[field] == value)