Update build_xml to use serialization

File is largely based on the existing custom_source example.
Uses the serialization attribute on the source to
point to an XML file
containing the serialized representation of the source.
Also builds the serialized_source.xml file with some default values.
Uses a new name for the library containing the serializable source.

examples/serialized_custom_source/build_xml.py

@@ -0,0 +1,46 @@
+import openmc
+
+# Define the serialised source
+serialized_source = """<Source>
+  <Radius>1.5</Radius>
+  <Energy>1e3</Energy>
+</Source>
+"""
+with open('serialized_source.xml', 'w') as f:
+    f.write(serialized_source)
+
+# Create a single material
+iron = openmc.Material()
+iron.set_density('g/cm3', 5.0)
+iron.add_element('Fe', 1.0)
+mats = openmc.Materials([iron])
+mats.export_to_xml()
+
+# Create a 5 cm x 5 cm box filled with iron
+box = openmc.model.rectangular_prism(10.0, 10.0, boundary_type='vacuum')
+cell = openmc.Cell(fill=iron, region=box)
+geometry = openmc.Geometry([cell])
+geometry.export_to_xml()
+
+# Tell OpenMC we're going to use our custom source
+settings = openmc.Settings()
+settings.run_mode = 'fixed source'
+settings.batches = 10
+settings.particles = 1000
+source = openmc.Source()
+source.library = 'build/libserialized_source.so'
+source.serialization = 'serialized_source.xml'
+settings.source = source
+settings.export_to_xml()
+
+# Finally, define a mesh tally so that we can see the resulting flux
+mesh = openmc.RegularMesh()
+mesh.lower_left = (-5.0, -5.0)
+mesh.upper_right = (5.0, 5.0)
+mesh.dimension = (50, 50)
+
+tally = openmc.Tally()
+tally.filters = [openmc.MeshFilter(mesh)]
+tally.scores = ['flux']
+tallies = openmc.Tallies([tally])
+tallies.export_to_xml()