Added support for elements with weight density

This commit also includes a Python script, data file, and template that can automatically generate the expand_natural_element subroutine for future development.
Added test for natural element wo.
Fixed whitespace
Added the Os natural element exception
Simplified the usage of write_atomic_weights.py
Improved documentation for write_atomic_weights.py
Merge branch 'develop' into element_wo
Merge branch 'element_wo' into hex_lattice_bug02