Turn the nf-core modules into a workflow: sourmash sketch & sourmash gather

[taylorreiter]

This PR integrates the sourmash sketch and sourmash gather modules into a DSL2 workflow, tacking on after fastqc and multiqc. I use the nf-core sketch and gather modules, and create my own module to download the GTDB reps and human (k = 21) databases/signatures. I hard coded this for now because these should be our defaults, but in the future it could be cool to parameterize the whole workflow on ksize if there's a need for that.

One thing I haven't figured out is how to make the contamination database small for test runs. Right now, I have it running against a human genome signature and all of GTDB reps. This is way to big for a test run. The test should just run against human.

PR checklist

• [x] 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).
• [x] 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).

[taylorreiter]

I'm going to close this PR because I implemented this work over in #8, but re-arranged a bunch of branches and stuff.