Expand Snakemake material

[jhidding]

Currently (without the cosmology example) the Snakemake part is a bit thin. We should at least explain wildcards and the expand function.

[ocaisa]

There are couple of lesson in the incubator already that cover snakemake in quite a bit of detail:
https://github.com/carpentries-incubator/hpc-workflows (recent fork on which we, HPC Carpentry, plan further development)
https://github.com/carpentries-incubator/workflows-snakemake (origin of HPC Carpentry fork, last commit almost 2 years ago)
https://github.com/carpentries-incubator/snakemake-novice-bioinformatics/ (active)