adding test_pipeline to see flow and ease debugging

[vroulet]

See issue #21.

The point is to add a pipeline test to

• ease debugging base functions (typically this would ease interactive debugging),
• give a good idea of the flow directly in the template.

The test is very basic and intended so for people to modify it according to their needs.
Note: I had to change python-gd.py to python_gd.py

@mathurinm what do you think?