I searched and replaced all molecular_weight and replaced them with molecular_mass
updating comments
updating comments and adding back liscense.txt
fixed more incorrect usages of name molecular weight
undid changes to material_compendium
Fixed merging issues in nuc_data_make.py
Fixed merging issues in nuc_data_make.py
Fixed naming issue in nuc_data_make with incorrect import name
replaced atomic_mass.py with atomic_weight.py and renamed atomic_weight.py to atomic_mass.py
fixed indentation
Merge branch 'howland-weightToMass' into staging
Merge branch 'staging' of github.com:pyne/pyne into staging
some final weight -> mass fixes