Add detailed compiler documentation

[fschueler]

To make the later maintenance of the Flink-backend easier, we should document the compiler details while we figure them out. Maybe we should even shortly describe the instructions - and possibly do a better job with the JavaDoc there than it's the case in SystemML right now. I suggest to

• Keep a Google-doc / Wiki-page with details about the compiler.
• Key classes, key points in the Flink implemenation, how does it work in general...
• I started a high level overwiew a while ago that can be found here.
• Add JavaDoc to all instruction classes
  • Currently not all instruction classes in SystemML have sufficient documentation to understand right away what they do.
  • Additionally, some instructions perform recompilation inside their executeInstruction methods. this can lead to the effect that a SparkInstruction never actually executes a Spark job. I think it would be good to have this information somewhere.