global: new directory structure

Restructure this REANA demo example to better separate input data, environment specification, analysis code, workflow definition, and other miscellanea such as documentation.

Use separate directories such as:

• code/foo.py
• data/input.dat
• documentation/notes.txt
• documentation/screenshot.png
• environment/Dockerfile
• workflow/workflow.cwl

We can also advertise the usage of separate directories for results and log files:

• results/myresult.cvs
• results/myplot.cvs
• logs/stdout.txt
• logs/stderr.txt
• logs/profile.txt

Use similar structure as in https://github.com/reanahub/reana-demo-helloworld/issues/16