helloworld: example simplification

[tiborsimko]

• Removes unnecessary files, environment variables, copyright messages etc in order to simplify the hello world example. Adds documentation.

• Amends program parameters and removes unnecessary environment variables.

• Amends container image to add entrypoint.

• Renames yadage workflow spec file to match the example name.

Signed-off-by: Tibor Simko tibor.simko@cern.ch

[tiborsimko]

@diegodelemos @hjhsalo This is the example simplification I had in mind. A necessary change for reana will come in another PR. Please have a look.

Note that a further simplification will come when we shall have stdout/stderr capturing. Then we can remove the outputfile handling from the hello world example, since the input/output is more appropriately demonstrated in the reana-demo-worldpopulation example.

Even later on, we could remove the workflow specs, so that the "hello world" would remain as a simple "run this job" kind of example, as opposed to "run this analysis" kind of examples.

[diegodelemos]

I will merge this (we can move docs discussion to a new PR if necessary) since these changes will make the example work.