New class: DifferentialGeometry
New class: Shape
New function: quadratic(A, B, C)
Made main.py executable
Made sure Emacs temp files are ignored by git
Added unit tests for pbrt helper functions: round_up_pow_2, lerp, eq, clamp, quadratic
Added a copy constructor for Transform.
Made sure the matrices are copied when passed to the default constructor
New class: Sphere
Added a unit test for Transform.inverse() method.
Added unit tests for class Sphere
Added timing script for computations of intersections with a Sphere.