factor out snakemake part into hpc-workflow

[psteinb]

I am bound to teach snakemake at my place. People have expressed high interest, so I just went through the current state of the material. I think it contains a lot of gems and is well written. Thanks to all the authors.

However, I have the feeling that workflows are so important, that they potentially need a repo for their own. I could imagine refactoring the snakemake focussed modules into hpc-carpentry/hpc-workflow. I just wanted to put this idea out there and hope that people comment on this.