Beam python nexmark benchmark starter task

For this starter task, the assignee is expected to

• get familiar with beam model by browse through the beam website and programming guide: https://beam.apache.org/

• set up development environment: https://cwiki.apache.org/confluence/display/BEAM/Developer+Guides

• also get some familiarity with the NexMark suite:
  http://datalab.cs.pdx.edu/niagara/pstream/nexmark.pdf

• it is also recommended to spend some time to get familiar with the git workflow