code: def nnz_obs(self):
          
          nnz = 0
          for w in self.observation_data.weight:
              if w > 0.0:
                  nnz += 1
          return nnz
docs: get the number of non-zero weighted observations
  
          Returns
          -------
          nnz_obs : int
              the number of non-zeros weighted observations