Add different natural element expansions.
Add <natural_elements> option in settings.xml.
Merge branch 'develop' into expand-natural
Add support for JENDL natural elements.
Added documentation on <natural_elements>.
Fix natural element expansion for Zn to match IUPAC 2009.
Add link to IUPAC 2009 document.
Add <natural_elements> tag in test_natural_element/settings.xml.
Merge branch 'develop' into expand-natural
Merge pull request #270 from paulromano/expand-natural

Determine natural element expansion based on cross section library