Fix test assertions for scalar temperature/density in Cell.from_xml_element

Cell.from_xml_element now converts single-element lists to scalars
(via `elif len(values) == 1: values = values[0]`), so temperature and
density round-trip as scalars when a single value is specified. Update
test assertions accordingly and fix XML serialization check to use
attribute lookup instead of sub-element for scalar temperature.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pshriwise

tests/unit_tests/dagmc/test_model.py

@@ -352,14 +352,14 @@ def test_dagmc_xml_temperature_roundtrip():
 
     dag_univ = openmc.DAGMCUniverse.from_xml_element(elem, mats)
     assert dag_univ.cells[7].fill.id == 1
-    assert dag_univ.cells[7].temperature == pytest.approx([825.0])
+    assert dag_univ.cells[7].temperature == pytest.approx(825.0)
 
     root = ET.Element('geometry')
     dag_univ.create_xml_subelement(root)
     dagmc_elem = root.find('dagmc_universe')
     xml_cell = dagmc_elem.find('cell')
-    assert xml_cell.find('temperature').text == '825.0'
+    assert xml_cell.get('temperature') == '825.0'
 
     dag_univ_roundtrip = openmc.DAGMCUniverse.from_xml_element(dagmc_elem, mats)
     assert dag_univ_roundtrip.cells[7].fill.id == 1
-    assert dag_univ_roundtrip.cells[7].temperature == pytest.approx([825.0])
+    assert dag_univ_roundtrip.cells[7].temperature == pytest.approx(825.0)

tests/unit_tests/test_cell.py

@@ -391,8 +391,8 @@ def test_dagmccell_from_xml_element():
     assert cell.id == 5
     assert cell.name == 'fuel'
     assert cell.fill is mat
-    assert cell.density == [10.5]
-    assert cell.temperature == [900.0]
+    assert cell.density == 10.5
+    assert cell.temperature == 900.0
 
     # volume round-trip
     xml = '<cell id="6" material="1" volume="42.0"/>'