small fix for structured error covariance matrices

Author: pdevis

obsarray/unc_accessor.py

@@ -637,7 +637,8 @@ def structured_err_cov_matrix(self):
 
         covs_sum = np.zeros(structured_err_cov_matrix.shape)
         for unc in self:
-            covs_sum += unc[self._sli].err_cov_matrix().values
+            if unc.is_structured:
+                covs_sum += unc[self._sli].err_cov_matrix().values
 
         structured_err_cov_matrix.values = covs_sum