Add snakemake minimum version and linting

Improve the snakefile by adding a minimum Snakemake version (now tested with 5.10.0) and use Snakemake's built-in linting function for a standard layout.

Also see: https://snakemake.readthedocs.io/en/stable/snakefiles/writing_snakefiles.html?highlight=linter#best-practices