add Restricting DESeq2 to using 4 cores per job

[colindaven]

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- [ ] If necessary, also make a PR on the nf-core/differentialabundance branch on the nf-core/test-datasets repository.
• 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).

[github-actions]

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• Do not close this PR
• Click Edit and change the base to dev
• This CI test will remain failed until you push a new commit

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Hi @colindaven,

It looks like this pull-request is has been made against the colindaven/differentialabundance master branch.
The master branch on nf-core repositories should always contain code from the latest release.
Because of this, PRs to master are only allowed if they come from the colindaven/differentialabundance dev branch.

You do not need to close this PR, you can change the target branch to dev by clicking the "Edit" button at the top of this page.
Note that even after this, the test will continue to show as failing until you push a new commit.

Thanks again for your contribution!

[pinin4fjords]

closing in favour of #173