code: def draw(self,cov,num_reals):
          
          super(ObservationEnsemble,self).draw(cov,num_reals,
                                               names=self.pst.nnz_obs_names)
          self.loc[:,self.names] += self.pst.observation_data.obsval
docs: draw realizations of observation noise and add to mean_values
          Note: only draws noise realizations for non-zero weighted observations
          zero-weighted observations are set to mean value for all realizations
  
          Parameters
          ----------
          cov : pyemu.Cov
              covariance matrix that describes the support volume around the
              mean values.
          num_reals : int
              number of realizations to draw