Add homopolish for nanopore-only assembly

[Gilbaja]

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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!
• [x] If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• [ ] If necessary, also make a PR on the nf-core/bacass branch on the nf-core/test-datasets repository.
• [x] Make sure your code lints (nf-core pipelines lint).
• [x] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• [x] Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• [ ] Usage Documentation in docs/usage.md is updated.
• [ ] Output Documentation in docs/output.md is updated.
• [x] CHANGELOG.md is updated.
• [x] README.md is updated (including new tool citations and authors/contributors).

Issue #66

Adds homopolish as a tool for polishing after polished by medaka.
Before executing homopolish it needs a bacteria sketch to be downloaded (3.3GB) and unzipped.
To do so, an additional module is created, and the workflow includes a condition for checking if it has been already downloaded in the output folder. It also includes a new parameter for forcing the download.

Test cmd

nextflow run . -profile test_long,conda --polish_method medaka_homopolish --outdir results

Pending tasks

• Adding tests
• Check with singularity