'Returns
Return user\'s preferences values as an array
Notes
This method is a particular method in MatrixDataModel'

def preference_values_from_user(self, user_id):

    user_id_loc = np.where((self._user_ids == user_id))
    if (not user_id_loc[0].size):
        raise UserNotFoundError
    preferences = self.index[user_id_loc]
    return preferences
