Header file for the Body class
Fixed issues with code
Added include for Vector3
Added comment with description of the class
Fixed Vector3 syntax
Changing milliseconds to seconds on dt
Calling mass, position, and velocity by references
Merge branch 'body' of https://github.com/kkleidal/6s096-HW4 into engine
pulled body branch.  Also, made basic physics engine and attempted to configure make file for 3 new object (2 engine classes and body class)