Prepare for scpca-nf release v0.5.1

[allyhawkins]

Steps for a new release of scpca-nf

Preparing for the release

• Are all of the issues planned for this release resolved? If there are any issues that are unresolved, mark this issue as blocked by those on ZenHub.
• Update code and documentation with the latest version number in the development branch:
  • nextflow.config
  • internal-instructions.md
  • external-instructions.md
• Test that the workflow is in good working order with nextflow run alexslemonade/scpca-nf -latest -r development
• File a PR from the development branch to the main branch. This should include all of the changes that will be associated with the next release.

Creating a release

• On the releases page, choose Draft a new release.
• In Choose a tag, type a new release number using semantic versioning (vX.X.X) (you did update the title of this issue to match, right?), then click Create a new tag: vX.X.X on publish.
• Write a description of the major changes in this release. You may want to start with the auto-generated release notes to save time.
• Optional: If not all issues have been addressed, save a draft to return to later.
• Publish the release!