Modify test_energy_laws to include Ta-181

Ta-181 is among a handful of nuclides that uses Kalbach-Mann systematics with
linear-linear interpolation.

tests/test_energy_laws/geometry.xml

@@ -1,7 +1,5 @@
 <?xml version="1.0"?>
 <geometry>
-
   <surface id="1" type="sphere" coeffs="0 0 0 100" boundary="vacuum"/>
   <cell id="1" material="1" region="-1" />
-
 </geometry>

tests/test_energy_laws/materials.xml

@@ -1,11 +1,11 @@
 <?xml version="1.0"?>
 <materials>
-
+  <default_xs>71c</default_xs>
   <material id="1">
     <density value="20" units="g/cc" />
-    <nuclide name="U-233" xs="71c" ao="1.0" />
-    <nuclide name="H-2" xs="71c" ao="1.0" />
-    <nuclide name="Na-23" xs="71c" ao="1.0" />
+    <nuclide name="U-233" ao="1.0" />
+    <nuclide name="H-2" ao="1.0" />
+    <nuclide name="Na-23" ao="1.0" />
+    <nuclide name="Ta-181" ao="1.0" />
   </material>
-
 </materials>

tests/test_energy_laws/results_true.dat

@@ -1,2 +1,2 @@
 k-combined:
-2.421637E+00 5.324089E-03
+2.130076E+00 1.938907E-03

tests/test_energy_laws/settings.xml

@@ -1,14 +1,11 @@
 <?xml version="1.0"?>
 <settings>
-
   <eigenvalue>
     <batches>10</batches>
     <inactive>5</inactive>
     <particles>1000</particles>
   </eigenvalue>
-
   <source>
     <space type="point" parameters="0. 0. 0." />
   </source>
-
 </settings>

tests/test_energy_laws/test_energy_laws.py

@@ -1,11 +1,20 @@
 #!/usr/bin/env python
 
-"""The purpose of this test is to provide coverage of Watt, N-body phase space,
-and evaporation energy distributions. The only nuclide that uses a Watt fission
-spectrum in ENDF/B-VII.1 is U-233. The only nuclide that has a reaction using
-the N-body phase space distribution is H-2(n.2n). Several nuclides have
-reactions with evaporation spectra. In this test, the material is composed of
-U-233, H-2, and Na-23 (which has several reactions with evaporation spectra.
+"""The purpose of this test is to provide coverage of energy distributions that
+are not covered in other tests. It has a single material with the following
+nuclides:
+
+U-233: Only nuclide that has a Watt fission spectrum
+
+H-2: Only nuclide that has an N-body phase space distribution, in this case for
+(n,2n)
+
+Na-23: Has an evaporation spectrum and also has reactions that have multiple
+angle-energy distributions, so it provides coverage for both of those
+situations.
+
+Ta-181: One of a few nuclides that has reactions with Kalbach-Mann distributions
+that use linear-linear interpolation.
 
 """