Add Python script to automatically download NNDC ACE format data

This PR adds a Python script in the data/ directory that automatically downloads and extracts ACE format data from the [NNDC](http://www.nndc.bnl.gov/endf/b7.1/acefiles.html) and places a properly-configued cross_sections.xml file in the proper place. All that is required of the user to get working data is to execute that script and then set their CROSS_SECTIONS environment variable.

@bhermanmit or @nelsonag -- can one of you review this?