Tons of changes

General
removed Hybrid Mode
reimplemented Reduce/GroupReduce - now special JavaGroupReduce(grouping)
with subsequent MapPartition
reimplemented Combine - now MapPartition that sorts/groups completely in
python; separate from GroupReduce
reimplemented CoGroup - now uses Custom Operator
fixed DefaultJoin/-Cross binding
BufferSize reduced to 64mb (was 256mb)

Java
simplified source/sink binding
merged function classes
simplified remaining classes
all operators are named

Python
removed In-/Outputformat abstraction
operators are partially configured during plan creation (combine/keys)
fixed several Python3 issues (probably not all)
datasets have proper access to tree structure

Others
disabled DistributedCache file deletion