Expand/ improve big data class tutorials

[in progress]

Plan:
-start with a detailed explanation of the WordCount example (+clearly layout the different possible implementations)
-Math exercise. Explain the Pi estimation theorem. Provide a basic (no map reduce) java implementation of the problem. Ask for a Flink implementation
-Keep the Tf-Idf example
-Turn the Triangle enumeration algo. example into a graph exercise.
-(Potencially) Also try another algorithm (eg. Apriori)