Default xs in materials.xml

[paulromano]

In many cases, if a user is using one cross section set, e.g. ENDF/B-VII, they will likely be using the same cross section on many different nuclide, in that case 70c. It would be convenient to have a default cross section tag that would be applied to any nuclide that doesn't have a cross section listed:

<default_xs>70c</default_xs>
<material id="1">
  <density value="1.0" units="g/cc" />
  <nuclide name="H-1"  ao="2.0" />
  <nuclide name="O-16" ao="1.0" />
</material>

[paulromano]

This was added in commit 4a3166.