SAD updates

[kpamela]

• [x] update class diagrams to include TDG and mappers
• [x] create a representation of our layered model
• [x] fix up the small descriptions in section 2

[Lordofsnails]

@jng11232 and i finished the Class diagrams and the layered model, plz revise or give us comments (althought pam and charles gave us the go already)

[Jng11232]

Everything is done !