Store k and uncertainty through depletion

Useful for benchmarking and other code to code comparisons.
The data is pulled right from the C api but now the uncertainties
are preserved and written to the depletion results file

openmc/deplete/operator.py

@@ -525,7 +525,8 @@ class Operator(TransportOperator):
         rates = self.reaction_rates
         rates[:, :, :] = 0.0
 
-        k_combined = openmc.capi.keff()[0]
+        # Get k and uncertainty
+        k_combined = openmc.capi.keff()
 
         # Extract tally bins
         materials = self.burnable_mats

openmc/deplete/results.py

@@ -237,8 +237,8 @@ class Results(object):
                               chunks=(1, 1, n_mats, n_nuc_rxn, n_rxn),
                               dtype='float64')
 
-        handle.create_dataset("eigenvalues", (1, n_stages),
-                              maxshape=(None, n_stages), dtype='float64')
+        handle.create_dataset("eigenvalues", (1, n_stages, 2),
+                              maxshape=(None, n_stages, 2), dtype='float64')
 
         handle.create_dataset("time", (1, 2), maxshape=(None, 2), dtype='float64')
 

openmc/deplete/results_list.py

@@ -95,7 +95,7 @@ class ResultsList(list):
 
         """
         time = np.empty_like(self, dtype=float)
-        eigenvalue = np.empty_like(self, dtype=float)
+        eigenvalue = np.empty((len(self), 2), dtype=float)
 
         # Get time/eigenvalue at each point
         for i, result in enumerate(self):