Changes

Summary

1. create material data models (details)
2. git add materials models to spar (details)
3. add foam material (details)
4. fix units error; delete htail override of skin material (details)
5. add init file to materials; fix vtail AR (details)
6. make material class attribute of material folder, global variable g (details)

create material data models

git add materials models to spar

add foam material

fix units error; delete htail override of skin material

add init file to materials; fix vtail AR

make material class attribute of material folder, global variable g