Replace the deprecated 'subworkflow' snakemake feature

[rgutzen]

snakemake >= 8.0.0 introduces breaking changes including dropping support for the deprecated 'subworkflow' feature which is used in cobrawap. To make use of the newer snakemake versions we must adopt the suggested alternative "module" feature to realize the same functionality.

[mdenker]

Next todo: make a real fix that does not rely on limiting the snakemake version.