Setting air to vacuum for beamline testing.

Should be temporary.
Note that athena does not have the same material definitions.

ip6/materials.xml

@@ -6,7 +6,7 @@
        http://www.engineeringtoolbox.com/air-composition-24_212.html
   -->
   <material name="Air">
-    <D type="density" unit="g/cm3" value="0.0012"/>
+    <D type="density" unit="g/cm3" value="0.0000000000012"/>
     <fraction n="0.754" ref="N"/>
     <fraction n="0.234" ref="O"/>
     <fraction n="0.012" ref="Ar"/>