Delegate Nuclide yield properties to FissionYieldDistribution

For consistency across the various fission yield helpers, the
Nuclide object now sets the yield data to always be a
FissionYieldDistribution or an empty dictionary.
Nuclide.yield_energies is now a property that returns the
energies from the FissionYieldDistribution.

When setting the yield_data, the Nuclide enforces the object
to be a Mapping instance. If the incoming yield_data is a
FissionYieldDistribution, no conversions are made. Otherwise,
a new distribution is created with the from_dict method.

The Chain.from_xml method now creates the yield_data for
nuclides using FissionYieldDistribution.from_dict

Changes were made to some unit tests that attempted to set
the yield_energies, which do not have a setter.

tests/unit_tests/test_deplete_chain.py

@@ -129,7 +129,7 @@ def test_from_xml(simple_chain):
     assert [r.branching_ratio for r in nuc.reactions] == [1.0, 0.7, 0.3]
 
     # Yield tests
-    assert nuc.yield_energies == [0.0253]
+    assert nuc.yield_energies == (0.0253, )
     assert list(nuc.yield_data) == [0.0253]
     assert nuc.yield_data[0.0253].products == ("A", "B")
     assert (nuc.yield_data[0.0253].yields == [0.0292737, 0.002566345]).all()
@@ -163,7 +163,6 @@ def test_export_to_xml(run_in_tmpdir):
         nuclide.ReactionTuple("(n,gamma)", "A", 0.0, 0.7),
         nuclide.ReactionTuple("(n,gamma)", "B", 0.0, 0.3)
     ]
-    C.yield_energies = [0.0253]
     C.yield_data = nuclide.FissionYieldDistribution.from_dict({
         0.0253: {"A": 0.0292737, "B": 0.002566345}})
 

tests/unit_tests/test_deplete_fission_yields.py

@@ -19,13 +19,11 @@ def nuclide_bundle():
         1.40e7: {"Xe135": 4.54e-3, "Gd155": 5.83e-8}}
     u235 = Nuclide()
     u235.name = "U235"
-    u235.yield_energies = (0.0253, 5.0e5, 1.40e7)
     u235.yield_data = FissionYieldDistribution.from_dict(u5yield_dict)
 
     u8yield_dict = {5.00e5: {"Xe135": 1.12e-3, "Gd155": 1.32e-12}}
     u238 = Nuclide()
     u238.name = "U238"
-    u238.yield_energies = (5.00e5,)
     u238.yield_data = FissionYieldDistribution.from_dict(u8yield_dict)
 
     xe135 = Nuclide()

tests/unit_tests/test_deplete_nuclide.py

@@ -3,6 +3,7 @@
 import xml.etree.ElementTree as ET
 
 import numpy
+import pytest
 
 from openmc.deplete import nuclide
 
@@ -73,8 +74,12 @@ def test_from_xml():
     ]
     expected_yield_data = nuclide.FissionYieldDistribution.from_dict({
         0.0253: {"Xe138": 0.0481413, "Zr100": 0.0497641, "Te134": 0.062155}})
-    assert u235.yield_energies == [0.0253]
     assert u235.yield_data == expected_yield_data
+    # test accessing the yield energies through the FissionYieldDistribution
+    assert u235.yield_energies == (0.0253, )
+    assert u235.yield_energies is u235.yield_data.energies
+    with pytest.raises(AttributeError):  # not settable
+        u235.yield_energies = [0.0253, 5e5]
 
 
 def test_to_xml_element():
@@ -91,7 +96,6 @@ def test_to_xml_element():
         nuclide.ReactionTuple('fission', None, 2.0e8, 1.0),
         nuclide.ReactionTuple('(n,gamma)', 'A', 0.0, 1.0)
     ]
-    C.yield_energies = [0.0253]
     C.yield_data = nuclide.FissionYieldDistribution.from_dict(
         {0.0253: {"A": 0.0292737, "B": 0.002566345}})
     element = C.to_xml_element()