add content for weeks 2 3 and 4.

exercises/exercise-2.md

exercises/exercise-3.md

labs/lab2-reductions-and-prefix-sums/README.md

labs/lab2-reductions-and-prefix-sums/angle-example-2.svg

labs/lab2-reductions-and-prefix-sums/angle-example.svg

labs/lab2-reductions-and-prefix-sums/angle.png

labs/lab2-reductions-and-prefix-sums/angle2.png

labs/lab2-reductions-and-prefix-sums/terrain.png

labs/lab2-reductions-and-prefix-sums/terrain.svg

labs/lab2-reductions-and-prefix-sums/visibility.png

labs/lab2-reductions-and-prefix-sums/visibility.svg

labs/lab3-k-means/README.md

labs/lab3-k-means/clusters.png

labs/lab3-k-means/clusters.svg

labs/lab3-k-means/points.png

labs/lab3-k-means/points.svg

labs/lab3-k-means/start.png

labs/lab3-k-means/step0.png

labs/lab3-k-means/step0.svg

labs/lab3-k-means/step1.png