Fixed XML Reading Input on Multiple Levels of Tags

When using XML-DOM, when you ask for a list of nodes of a certain tagname, it will search through immediate children and recursively their children. This can cause errors in the way we want OpenMC input files to work. @nelsonag noticed that if ```<cross_sections>``` tagname is used in ```<output>``` as an element node and not an attribute, OpenMC will also see this node when searching for the cross_section directory file path. This is stated in issue #228 .

With this pull request a new function was implemented in FoX-DOM called ```getChildrenByTagName```. It is exactly the same as the DOM standard routine ```getElementsByTagName```, except that before pointing to another child element, it checks to make sure that the parent node is the node that we sent the routine. Therefore if you send a pointer that is on the hierarchical level of ```<settings>```  in the following input file:

```xml
<settings>

  <output>
     <cross_sections> true </cross_sections>
  </output>

  <cross_sections> path </cross_sections>

</settings>
```

The only nodes that getChildrenByTagName will see is ```<output>``` and the second ```<cross_sections>```. It will return *path* now instead of *true*.

Also in the PR, a flag was added for compiling FoX-DOM on BG/Q.