Create Existence020.java

I added a variable to store the previousInteraction. And I created two methods to set and get this.
I utilized the incSelfSatisfactionCounter from the previous lesson.

I tried adding in randomness or adding new interactions to make it a bit more interesting. You kinda have to rewrite the code all the way down to get the new experiments/interactions actually working (I didn't, figuring we'd do something like it later... I hope.)