Started writing script to download ACE files from NNDC site.
Check for file already downloaded. Rename xsdir.
Merge branch 'develop' into nndc-data
Extract files to subdirectories. Add pre-generated cross_sections.xml.
Update installation guide with information about NNDC data.
Merge branch 'develop' into nndc-data
Updated data/readme.rst.
Ask user to delete downloaded tgz files.
Make get_nndc_data.py compatible with Python 3.
Merge pull request #240 from mit-crpg/nndc-data

Add Python script to automatically download NNDC ACE format data