Scaffolding Workflow: basic workflow for yahs

Among other tasks to consider:

synchronise import of nf-core modules with the polishing branch (currently relies on local import)
there is a bioconda package for yahs. Can it be converted into docker/singularity image?
gfastats is currently used from PATH (https://github.com/vgl-hub/gfastats). Could it be wrapped into package or made optional?

PR checklist

[ ] This comment contains a description of changes (with reason).
[ ] If you've fixed a bug or added code that should be tested, add tests!
[ ] If you've added a new tool - have you followed the pipeline conventions in the contribution docs
[ ] Make sure your code lints (nf-core lint).
[ ] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
[ ] Usage Documentation in docs/usage.md is updated.
[ ] Output Documentation in docs/output.md is updated.
[ ] CHANGELOG.md is updated.
[ ] README.md is updated (including new tool citations and authors/contributors).